@@ -299,8 +299,8 @@ object.
 
    A bytes writer instance.
 
-   The API is **not thread safe**: a writer should only be used by a single
-   thread at the same time.
+   The API is **not thread safe**. To share a writer with multiple threads, a
+   critical section or a lock is needed.
 
    The instance must be destroyed by :c:func:`PyBytesWriter_Finish` on
    success, or :c:func:`PyBytesWriter_Discard` on error.
@@ -436,7 +436,7 @@ Low-level API
    On success, return ``0``.
    On error, set an exception and return ``-1``.
 
-   *size* can be negative to shrink the writer.
+   *grow* can be negative to shrink the writer.
 
 .. c:function:: void* PyBytesWriter_GrowAndUpdatePointer(PyBytesWriter *writer, Py_ssize_t size, void *buf)
 
 
@@ -16,6 +16,13 @@ points must be below 1114112 (which is the full Unicode range).
 
 UTF-8 representation is created on demand and cached in the Unicode object.
 
+.. impl-detail::
+
+   The internal buffer always includes an extra trailing null character for
+   compatibility with null terminated C strings. This extra character is not
+   counted in :c:func:`PyUnicode_GetLength` nor in the various *size* arguments
+   of the functions below.
+
 .. note::
    The :c:type:`Py_UNICODE` representation has been removed since Python 3.12
    with deprecated APIs.
@@ -164,11 +171,15 @@ access to internal read-only data of Unicode objects:
    .. versionadded:: 3.3
 
 
-.. c:function:: Py_UCS4 PyUnicode_READ(int kind, void *data, \
-                                       Py_ssize_t index)
+.. c:function:: Py_UCS4 PyUnicode_READ(int kind, void *data, Py_ssize_t index)
 
    Read a code point from a canonical representation *data* (as obtained with
-   :c:func:`PyUnicode_DATA`).  No checks or ready calls are performed.
+   :c:func:`PyUnicode_DATA`).  No checks are performed.
+
+   .. impl-detail::
+
+      Accept reading the trailing null character at index
+      :c:func:`PyUnicode_GetLength`.
 
    .. versionadded:: 3.3
 
@@ -179,6 +190,11 @@ access to internal read-only data of Unicode objects:
    representation.  This is less efficient than :c:func:`PyUnicode_READ` if you
    do multiple consecutive reads.
 
+   .. impl-detail::
+
+      Accept reading the trailing null character at index
+      :c:func:`PyUnicode_GetLength`.
+
    .. versionadded:: 3.3
 
 
@@ -409,6 +425,10 @@ APIs:
    the UCS1 range, it will be transformed into UCS1
    (:c:macro:`PyUnicode_1BYTE_KIND`).
 
+   All characters must be in range [U+0000; U+10ffff]. If *kind* is
+   :c:macro:`PyUnicode_4BYTE_KIND` and the string contains invalid characters,
+   the behavior is undefined.
+
    .. versionadded:: 3.3
 
 
@@ -716,6 +736,10 @@ APIs:
 
    On error, set an exception and return ``-1``.
 
+   .. impl-detail::
+
+      The length does not count the trailing null character.
+
    .. versionadded:: 3.3
 
 
@@ -794,6 +818,11 @@ APIs:
 
    Return character on success, ``-1`` on error with an exception set.
 
+   .. impl-detail::
+
+      Do not accept reading the trailing null character at index
+      :c:func:`PyUnicode_GetLength`.
+
    .. versionadded:: 3.3
 
 
@@ -1797,6 +1826,9 @@ object.
    The instance must be destroyed by :c:func:`PyUnicodeWriter_Finish` on
    success, or :c:func:`PyUnicodeWriter_Discard` on error.
 
+   The API is **not thread safe**. To share a writer with multiple threads, a
+   critical section or a lock is needed.
+
 .. c:function:: PyUnicodeWriter* PyUnicodeWriter_Create(Py_ssize_t length)
 
    Create a Unicode writer instance.
@@ -1868,10 +1900,13 @@ object.
 
 .. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, const Py_UCS4 *str, Py_ssize_t size)
 
-   Writer the UCS4 string *str* into *writer*.
+   Write the UCS4 string *str* into *writer*.
 
    *size* is a number of UCS4 characters.
 
+   All characters must be in range [U+0000; U+10ffff]. If the string contains
+   invalid characters, the behavior is undefined.
+
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
 
@@ -169,10 +169,10 @@ interpreted as prescribed by the ISO 8601 standard.  Year 0 is 1 BC, year -1 is
       the specified width, representing an empty day. The *weekday* parameter
       is unused.
 
-   .. method:: formatweek(theweek, w=0)
+   .. method:: formatweek(theweek, width)
 
-      Return a single week in a string with no newline. If *w* is provided, it
-      specifies the width of the date columns, which are centered. Depends
+      Return a single week in a string with no newline. *width* specifies
+      the width of the date columns, which are centered. Depends
       on the first weekday as specified in the constructor or set by the
       :meth:`setfirstweekday` method.
 
@@ -200,7 +200,7 @@ interpreted as prescribed by the ISO 8601 standard.  Year 0 is 1 BC, year -1 is
       :meth:`setfirstweekday` method.
 
 
-   .. method:: formatmonthname(theyear, themonth, width=0, withyear=True)
+   .. method:: formatmonthname(theyear, themonth, width, withyear=True)
 
       Return a string representing the month's name centered within the
       specified *width*. If *withyear* is ``True``, include the year in the
 
@@ -165,11 +165,13 @@ test suite, thanks to the :option:`-m` option: :program:`python -m test`. Under
 the hood, it uses :mod:`!test.regrtest`; the call :program:`python -m
 test.regrtest` used in previous Python versions still works.  Running the
 script by itself automatically starts running all regression tests in the
-:mod:`!test` package. It does this by finding all modules in the package whose
-name starts with ``test_``, importing them, and executing the function
-:func:`test_main` if present or loading the tests via
-unittest.TestLoader.loadTestsFromModule if ``test_main`` does not exist.  The
-names of tests to execute may also be passed to the script. Specifying a single
+:mod:`!test` package. It does this by finding modules and packages whose names
+start with ``test_``, importing each one, and loading its tests with
+:meth:`unittest.TestLoader.loadTestsFromModule`. See the
+:ref:`load_tests protocol <load_tests-protocol>` for how a module or package
+can customize what is loaded. Defining a module-level ``test_main()`` function
+is not supported and causes the test to fail. The names of tests to
+execute may also be passed to the script. Specifying a single
 regression test (:program:`python -m test test_spam`) will minimize output and
 only print whether the test passed or failed.
 
@@ -881,9 +883,11 @@ The :mod:`!test.support` module defines the following functions:
 
 .. function:: reap_children()
 
-   Use this at the end of ``test_main`` whenever sub-processes are started.
-   This will help ensure that no extra children (zombies) stick around to
-   hog resources and create problems when looking for refleaks.
+   Call this from :meth:`~unittest.TestCase.setUp`,
+   :meth:`~unittest.TestCase.tearDown`, or ``tearDownModule()`` in tests
+   that start subprocesses. This will help ensure that no extra children
+   (zombies) stick around to hog resources and create problems when looking
+   for refleaks.
 
 
 .. function:: get_attribute(obj, name)