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>