tweak comment

From-SVN: r44942

gcc/except.c

@@ -834,7 +834,11 @@ expand_end_all_catch ()
 
 /* End an exception region for an exception type filter.  ALLOWED is a
    TREE_LIST of types to be matched by the runtime.  FAILURE is an
-   expression to invoke if a mismatch ocurrs.  */
+   expression to invoke if a mismatch ocurrs.
+
+   ??? We could use these semantics for calls to rethrow, too; if we can
+   see the surrounding catch clause, we know that the exception we're
+   rethrowing satisfies the "filter" of the catch type.  */
 
 void
 expand_eh_region_end_allowed (allowed, failure)