Improve the documentation of `fnmatch`

[picnixz]

The implementation of fnmatch.fnmatch calls os.path.normcase on both the filename and the pattern, also allowing path-like objects. On the other hand, fnmatch.filter call os.path.normcase to normalize the pattern as well as os.path.normcase on the filenames being iterated over, but only on non-POSIX platforms.

Following the discussion on #123215, we decided not to change the runtime behaviour (i.e. do not call os.fspath) but simply clarify the documentation. Inconsistencies remain between platforms where non-POSIX platforms will not get a TypeError if they use path-like objects in fnmatch.filter. We will also not document fnmatch.fnmatch as accepting path-like objects.

Related:

• Allow path-like objects in fnmatch.filter on POSIX platforms #123135
• Allow path-like objects in fnmatch.fnmatchcase #123215 (comment)
• fnmatch: support pathlib typeshed#12522

Linked PRs

• gh-123345: improve fnmatch docs #123346
• [3.12] gh-123345: improve fnmatch docs (GH-123346) #128775
• [3.13] gh-123345: improve fnmatch docs (GH-123346) #128776