Documenting that expandTo conserves mass not number of atoms

[john-science]

What is the change? Why is it being made?

There was some confusion that using the expandTo feature in the nuclide flags section of the blueprints does not conserve the number of atoms. But that is because it conserves the total most of the element instead. (And you can only do one.)

Now, this was mentioned in a docstring somewhere, but the request was made to warn people in the documentation. Which is fair, and this PR accomplishes that.

close #1817

SCR Information

Change Type: fixes

One-Sentence Rationale: People should know what kind is conserved when they use the expandTo feature in nuclide flags.

One-line Impact on Requirements: NA

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.
• The documentation is still up-to-date in the doc folder.
• The code style follows good practices.
• The dependencies are still up-to-date in pyproject.toml.