Update HOT README about when single-page vacuums happen.

src/backend/access/heap/README.HOT

-$PostgreSQL: pgsql/src/backend/access/heap/README.HOT,v 1.6 2010/04/23 23:21:44 rhaas Exp $
+$PostgreSQL: pgsql/src/backend/access/heap/README.HOT,v 1.7 2010/09/19 17:51:44 momjian Exp $
 
 Heap Only Tuples (HOT)
 ======================
@@ -246,6 +246,12 @@ scans reasonably sized, the maximum number of line pointers per page
 is arbitrarily capped at MaxHeapTuplesPerPage (the most tuples that
 could fit without HOT pruning).
 
+Effectively, space reclamation happens during tuple retrieval when the
+page is nearly full (<10% free) and a buffer cleanup lock can be
+acquired.  This means that UPDATE, DELETE, and SELECT can trigger space
+reclamation, but often not during INSERT ... VALUES because it does
+not retrieve a row.
+
 
 VACUUM
 ------