This was missing in commit 0f488dce

Closes https://github.com/MusicPlayerDaemon/MPD/issues/568

Closes https://github.com/MusicPlayerDaemon/MPD/issues/340

Closes https://github.com/MusicPlayerDaemon/MPD/issues/526

Closes https://github.com/MusicPlayerDaemon/MPD/issues/524

Closes https://github.com/MusicPlayerDaemon/MPD/issues/523

.. and not "UNIX domain socket.  Be consistent about the naming.

Clarification for https://github.com/MusicPlayerDaemon/MPD/issues/505

 * "lsinfo" argument is optional
 * "tagtypes disable" arguments are mandatory (typo)

Closes #410

I added this sentence in commit
5271e81e, but this was merely
documented the legacy status quo, which has always been undocumented
for old-style filters.

But for new filters, using "==" for sub strings was a surprising
"feature", which I removed in commit
ac0852b4.

This feature requires `libpcre`.

Closes #399

Closes #397

Fix links and lots of broken markup.