In python3.8 on macos and in all versions on windows the multiprocessing
method is `spawn` which does not preserve class state.

The `normalize_paths()` utility was doing too much — parsing
unstructured configuration path data and dispatching the scrubbed paths
to be normalized.

Towards moving the parsing of unstructured configuration path data
closer towards were configuration occurs, have the utility accept only
structured input for normalizing paths.

This is the initial incision point to only accept `str` (or `None`) for
parsing out comma/whitespace/regexp separated values.

Certain versions of Python on Windows are capable of using
multiprocessing safely and correctly. Instead of completely disabling
multiprocessing on Windows, this allows us to check for safe versions.

This also updates our internal utility documentation to add missing
functions and add a strongly worded warning about the API of those
functions.

- Add more documentation around utils functions
- Fix documentation about default formatting plugins
- Add extra documentation of filenames_from predicate parameter
- Add test for the default parameter of flake8.utils.fnmatch