docs: fs: locks.rst: update comment about mandatory file locking

The mandatory file locking got removed due to its problems, but
the fs locks documentation still points to it.

Update the text there, informing that it was removed on Kernel
5.14.

Fixes: f7e33bdbd6d1 ("fs: remove mandatory file locking support")
Reported-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>

Documentation/filesystems/locks.rst

···
 1.3 Mandatory Locking As A Mount Option
 ---------------------------------------
 
-Mandatory locking, as described in
-'Documentation/filesystems/mandatory-locking.rst' was prior to this release a
-general configuration option that was valid for all mounted filesystems.  This
-had a number of inherent dangers, not the least of which was the ability to
-freeze an NFS server by asking it to read a file for which a mandatory lock
-existed.
+Mandatory locking was prior to this release a general configuration option
+that was valid for all mounted filesystems.  This had a number of inherent
+dangers, not the least of which was the ability to freeze an NFS server by
+asking it to read a file for which a mandatory lock existed.
 
-From this release of the kernel, mandatory locking can be turned on and off
-on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
-The default is to disallow mandatory locking. The intention is that
-mandatory locking only be enabled on a local filesystem as the specific need
-arises.
-
+Such option was dropped in Kernel v5.14.