Fix obsolete comment.

Comment about MaxAllocSize was not updated when the TOAST-header macros
were replaced in 8.3 "varvarlena" changes.  Per report from Frederik Ramm.

src/include/utils/memutils.h

@@ -30,7 +30,9 @@
  *		be summarily denied.
  *
  * XXX This is deliberately chosen to correspond to the limiting size
- * of varlena objects under TOAST.	See VARATT_MASK_SIZE in postgres.h.
+ * of varlena objects under TOAST.	See VARSIZE_4B() and related macros
+ * in postgres.h.  Many datatypes assume that any allocatable size can
+ * be represented in a varlena header.
  *
  * XXX Also, various places in aset.c assume they can compute twice an
  * allocation's size without overflow, so beware of raising this.