Enhance os.scandir() doc

Enhance os.scandir() doc

Issue #26248, patch written by Ben Hoyt:

1) Clarify that the return values of is_dir()/is_file()/etc are cached
   separately for follow_symlinks True and False.
2) Be more specific about when the functions require a system call, and how it
   relates to caching and follow_symlinks.
3) DRY up common stuff between is_dir and is_file by saying "Caching, system
   calls made, and exceptions raised are as per is_dir" in is_file.
4) Tweak to the first paragraph of docs for is_dir/is_file to simplify: assume
   the follow_symlinks=True default, then note the follow_symlinks=False
   non-default case after.