Index: gcc/doc/gty.texi
===================================================================
--- gcc/doc/gty.texi	(revision 161983)
+++ gcc/doc/gty.texi	(working copy)
@@ -183,6 +183,7 @@ The value of the expression given by @code{desc} i
 each @code{tag} value, each of which should be different.  If no
 @code{tag} is matched, the field marked with @code{default} is used if
 there is one, otherwise no field in the union will be marked.
+A union field can have at most one @code{tag}.
 
 In the @code{desc} option, the ``current structure'' is the union that
 it discriminates.  Use @code{%1} to mean the structure containing it.