cifs: create a new Documentation/ directory and move docfiles into it

Currently, we have a number of documentation files that live under
fs/cifs/. Generally, these don't get picked up by distro packagers,
since they're in a non-standard location. Move them to a new spot
under Documentation/ instead.
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NSteve French <smfrench@gmail.com>