"The documentation for mpd.conf still links to https://replaygain.org
– according to archive.org, this domain has been defunct since
2014-09-14, shortly after got domain squatted and ever since hosts
dubious content."

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1050

Update description with content from user manual [0].
Remove obsolete information regarding "required parameters" and log_level

[0] https://www.musicpd.org/doc/html/user.html#the-configuration-file

"DEFAULT" is a bad name - all it says is that it's the default value,
but it doesn't say what it means.  The name NOTICE mimics the syslog
level.

Calling this "secure" never made sense.  Messages about client
connects are just a small part of what gets logged as "secure",
a.k.a. "info".

Build with `rst2man mpd.1.rst mpd.1`