The writing and reading of metadata involves lots of different programs
and libraries. Therefore it is prudent to point out how exactly MPD
receives metadata. Ideally this helps to point users to the right place
if their tags are not picked up correctly.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1337

This enhances readability in sidebar, especially for "User’s Manual" and
"Protocol" pages

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1328

Add an option to the UPnP database plugin to configure which interface
is used by upnp to discover servers.

upnp by default selects the first interface that is not loopback, which
in some cases might not be the desired interface. For example if wanting
to access a DLNA server over a VPN connection.

The "interface" option can now be set to the name of the desired
interface to achieve this.

The default behaviour remains unchanged.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1106

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1285

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297

This commit also increases the PROTOCOL_VERSION so clients can detect
the availability of the feature.

Whoops, I misplaced this one.

Another one from https://github.com/MusicPlayerDaemon/MPD/issues/888

Closes https://github.com/MusicPlayerDaemon/MPD/issues/888

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1115

Some users want certain clients to fully control playback, but do not
want them to be able to trigger database update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1124

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

The implementation was removed 12 years ago in commit f6455d5f and
nobody missed it.

The existing implementation has been utterly broken forever; I cannot
explain what it actually does, but it doesn't do what the
documentation says.

Now, "+0" means "right after the current song" and "-0" means "right
before the current song".  Mnemonic: there are zero songs between the
current song and the newly added song.