smb3: move Documentation/filesystems/cifs to Documentation/filesystems/smb

Documentation/filesystems/cifs contains both server and client information
so its pathname is misleading.  In addition, the directory fs/smb
now contains both server and client, so move Documentation/filesystems/cifs
to Documentation/filesystems/smb

Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>