cifs: update Kconfig description

There were various outdated or missing things in fs/cifs/Kconfig
e.g. mention of support for insecure NTLM which has been removed,
and lack of mention of some important features. This also shortens
it slightly, and fixes some confusing text (e.g. the SMB1 POSIX
extensions option).

Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Acked-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>