Documentation/admin-guide/cifs: document open_files and dfscache

Add missing documentation for open_files and dfscache /proc files.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>