pgsql: Add UPDATE entry for row reuse.

Log Message:
-----------
Add UPDATE entry for row reuse.

* Allow heap reuse of UPDATEd rows if old and new versions are on the
same heap page?

This is possible for same-page updates because a single index row
can point to both old and new values.
http://archives.postgresql.org/pgsql-hackers/2006-06/msg01305.php

Modified Files:
--------------
pgsql/doc:
TODO (r1.1896 -> r1.1897)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1896&r2=1.1897)
pgsql/doc/src/FAQ:
TODO.html (r1.399 -> r1.400)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.399&r2=1.400)