pgsql: Update entry: < * Allow heap reuse of UPDATEd rows if old and

Log Message:
-----------
Update entry:

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

* Allow heap reuse of UPDATEd rows if no indexed columns are changed,
and 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.

While vacuum handles DELETEs fine, updating of non-indexed columns, like
counters, are difficult for VACUUM to handle efficiently. This method
is possible for same-page updates because a single index row can be
used to point to both old and new values.

Modified Files:
--------------
pgsql/doc:
TODO (r1.1897 -> r1.1898)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1897&amp;r2=1.1898)
pgsql/doc/src/FAQ:
TODO.html (r1.400 -> r1.401)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.400&amp;r2=1.401)