cifsd: fix WARNING: document isn't included in any toctree

Stephen reported a warning message from cifsd.rst file.

 Documentation/filesystems/cifs/cifsd.rst: WARNING: document isn't
 included in any toctree

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>