Some documentation tweaks

Add links to the D-Bus docs in some places, and various
other additions.

gio/gdbusmethodinvocation.c

@@ -43,6 +43,10 @@
  * Instances of the #GDBusMethodInvocation class are used when
  * handling D-Bus method calls. It provides a way to asynchronously
  * return results and errors.
+ *
+ * The normal way to obtain a #GDBusMethodInvocation object is to receive
+ * it as an argument to the handle_method_call() function in a
+ * #GDBusInterfaceVTable that was passed to g_dbus_connection_register_object().
  */
 
 struct _GDBusMethodInvocationPrivate