backends: Silence a couple of gtk-doc warnings

A lot of fancy gtk-doc documentation was added, but they had a couple of
issues: invalid return value annotations and wrong documented function
name.

src/backends/meta-monitor-manager.c

@@ -2580,7 +2580,7 @@ meta_monitor_manager_get_monitors (MetaMonitorManager *manager)
 }
 
 /**
- * meta_monitor_manager_get_capabilities:
+ * meta_monitor_manager_add_gpu:
  * @manager: A #MetaMonitorManager object
  *
  * Should only be called by subclasses. Adds a #MetaGpu to the internal list of

src/backends/meta-renderer.c

@@ -131,7 +131,7 @@ meta_renderer_set_legacy_view (MetaRenderer     *renderer,
  * Returns a list of #MetaRendererView objects, each dealing with a part of the
  * stage.
  *
- * Returns: (transfer-none) (element-type MetaRendererView): a list of
+ * Returns: (transfer none) (element-type MetaRendererView): a list of
  * #MetaRendererView objects.
  */
 GList *