doc/user: add more documentation for the proxy database plugin

doc/mpdconf.example

@@ -175,6 +175,15 @@
 ###############################################################################
 
 
+# Database #######################################################################
+#
+
+#database {
+#       plugin "proxy"
+#       host "other.mpd.host"
+#       port "6600"
+#}
+
 # Input #######################################################################
 #
 

doc/user.xml

@@ -714,7 +714,10 @@ systemctl start mpd.socket</programlisting>
 
         <para>
           Provides access to the database of another MPD instance
-          using <filename>libmpdclient</filename>.  Experimental!
+          using <filename>libmpdclient</filename>.  This is useful
+          when you run mount the music directory via NFS/SMB, and the
+          file server already runs a MPD instance.  Only the file
+          server needs to update the database.
         </para>
 
         <informaltable>