Switch to gtk-doc syntax

https://bugzilla.gnome.org/show_bug.cgi?id=673752

src/meta/common.h

@@ -364,6 +364,7 @@ void meta_frame_borders_clear (MetaFrameBorders *self);
   (ycoord) <  ((rect).y + (rect).height))
 
 /**
+ * MetaStackLayer:
  * Layers a window can be in.
  * These MUST be in the order of stacking.
  */