Fix misleading comment in paraminfo_get_equal_hashops

The comment mistakenly claimed the code was checking PlaceHolderVars for
volatile functions when the code was actually checking lateral vars.

Update the comment to reflect the reality of the code.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs48HZGZOV85g0fx8z1qDx6NNKHexJPT2FCnKnZhxBWkd-A@mail.gmail.com

src/backend/optimizer/path/joinpath.c

@@ -529,7 +529,7 @@ paraminfo_get_equal_hashops(PlannerInfo *root, ParamPathInfo *param_info,
 		Node	   *expr = (Node *) lfirst(lc);
 		TypeCacheEntry *typentry;
 
-		/* Reject if there are any volatile functions in PHVs */
+		/* Reject if there are any volatile functions in lateral vars */
 		if (contain_volatile_functions(expr))
 		{
 			list_free(*operators);