Update docs for metadata parameter of write_image

[keller-mark]

The ** for the **metadata kwarg was confusing to me. It wasn't initially apparent to me via the type anotations or docstring whether I should do write_image(omero={}) vs write_image(metadata={"omero":{}}). Only the latter works.

write_image(
    image=img_arr,
    # other args here
    metadata={
        "omero": {
            "name": img_name,
        }
    }
)

[jo-mueller]

@keller-mark I think this looks good, but since the merge of #515 you'll need to rebase. OR check out the new API introduced there, which should make writing the omero metadata a tad easier!