nixos/doc/matrix-synapse: refactor

* Linkify all service options used in the code-examples.
* Demonstrated the use of `riot-web.override {}`.
* Moved the example how to configure a postgresql-database for
  `matrix-synapse` to this document from the 20.03 release-notes.

nixos/modules/services/misc/matrix-synapse.nix

@@ -113,7 +113,7 @@ ${cfg.extraConfig}
 '';
 
   hasLocalPostgresDB = let args = cfg.database_args; in
-    usePostgresql && (!(args ? host) || (elem args.host [ "localhost" "127.0.0.1" ]));
+    usePostgresql && (!(args ? host) || (elem args.host [ "localhost" "127.0.0.1" "::1" ]));
 in {
   options = {
     services.matrix-synapse = {
@@ -721,4 +721,6 @@ in {
     '')
   ];
 
+  meta.doc = ./matrix-synapse.xml;
+
 }