Update comment for README.HOT

I would like to apply the attached patch to README.HOT so clarify when
single-page cleanup happens, e.g. not during INSERT.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
Index: src/backend/access/heap/README.HOT
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/heap/README.HOT,v
retrieving revision 1.6
diff -c -c -r1.6 README.HOT
*** src/backend/access/heap/README.HOT    23 Apr 2010 23:21:44 -0000    1.6
--- src/backend/access/heap/README.HOT    17 Sep 2010 19:48:16 -0000
***************
*** 246,251 ****
--- 246,257 ----
  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%) and a buffer cleanup lock can be acquired.
+ This means that UPDATE, DELETE, and SELECT can trigger space
+ reclamation, while INSERT ... VALUES cannot because it does not retrieve
+ a row.
+

  VACUUM
  ------