Add a check in FileSystem to ensure paths with Windows-style separators
aren't used. This won't detect mixed separators (e.g. from concatenating
two paths with different separators), but it will detect the more common
case of someone passing a complete Windows-style path. Such paths have
never been officially supported, and have always been partially broken in
FileSystem. This simply makes them fail fast(er).

Also improves docs to explain path formats more clearly, and updates older
docs on how FileSystem is initialized.
Merge pull request #6699 from adobe/pflynn/warn-win-paths

Fail fast on paths with Windows-style separators