nfs(5): Update description of sec= mount option

Bryan recently added SECINFO support, and I've beefed up the NFSv3
MNT processing in kernel to do some security flavor negotiation.

Thus the kernel can perform additional security flavor negotiation
now.  Update the description of the sec= mount option and the
SECURITY CONSIDERATIONS section to reflect this change.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>