man: document Unix domain socket configuration in the manual

doc/mpd.conf.5

@@ -72,6 +72,9 @@ This specifies the user that mpd will run as, if set.
 This specifies which address mpd binds to and listens on.  Multiple
 bind_to_address parameters may be specified.  The default is "any", which binds
 to all available addresses.
+
+To bind to a Unix domain socket, specify an absolute path.  For a
+system-wide MPD, we suggest the path "\fB/var/run/mpd/socket\fP".
 .TP
 .B port <port>
 This specifies the port that mpd listens on.  The default is 6600.

doc/mpdconf.example

@@ -42,6 +42,10 @@ error_file                      "~/.mpd/mpd.error"
 #bind_to_address                 "any"
 #port                            "6600"
 #
+# Bind to a Unix domain socket:
+#
+#bind_to_address                 "/var/run/mpd/socket"
+#
 # Controls the amount of information that is logged.  Can be
 # "default", "secure", or "verbose".
 #