8.4 doc bug for "Incrementally Updated Backups"

Based on Jürgen Fuchsberger's experience described on the general
list, I think the following should have been back-patched to 8.4 and
8.3 (assuming it gets a minor update before it EOL) as well, not just
9.0.

In particular, in 8.4 chapter 24.4.5 it says "it will be up to you to
determine how far back you need to keep WAL segment files to have a
recoverable backup" but it dangerously does not mention that it is
also up to you to construct a backup_label file such that those WAL
segment files will actually get used.

commit 13e6d6c5da184abcdfcfc9874ad17ef09f4ea044
Author: Bruce Momjian <bruce(at)momjian(dot)us>
Date: Wed Aug 25 23:55:54 2010 +0000

Remove docs for "Incrementally Updated Backups" because it was of
questionable reliability; information moved to a wiki:

http://wiki.postgresql.org/wiki/Incrementally_Updated_Backups

Backpatch to 9.0.

Cheers,

Jeff