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: f7e33bdb ("fs: remove mandatory file locking support")
Reported-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>