improve DECL_ABSTRACT_ORIGIN comment

From-SVN: r152404

gcc/tree.h

@@ -2484,10 +2484,11 @@ struct GTY(()) tree_decl_minimal {
 
 
 /* For any sort of a ..._DECL node, this points to the original (abstract)
-   decl node which this decl is an instance of, or else it is NULL indicating
-   that this decl is not an instance of some other decl.  For example,
-   in a nested declaration of an inline function, this points back to the
-   definition.  */
+   decl node which this decl is an inlined/cloned instance of, or else it
+   is NULL indicating that this decl is not an instance of some other decl.
+
+   The C front-end also uses this in a nested declaration of an inline
+   function, to point back to the definition.  */
 #define DECL_ABSTRACT_ORIGIN(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.abstract_origin)
 
 /* Like DECL_ABSTRACT_ORIGIN, but returns NODE if there's no abstract