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 <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

docs: filesystems: convert mandatory-locking.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Use notes markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/aecd6259fe9f99b2c2b3440eab6a2b989125e00d.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: convert locks.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/467b3f20e63d2640d22599b99229699b5fb79251.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>