pgsql: Update README.HOT to reflect new snapshot tracking and xmin

Log Message:
-----------
Update README.HOT to reflect new snapshot tracking and xmin advancement
code in 8.4.

Modified Files:
--------------
pgsql/src/backend/access/heap:
README.HOT (r1.3 -> r1.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/README.HOT?r1=1.3&r2=1.4)