pgsql: Add deprecation notice about planned removal in 8.4 and advice to

Log Message:
-----------
Add deprecation notice about planned removal in 8.4 and advice to try using
the newer XML stuff in core.

(This should probably also be referred to in the release notes.)

Modified Files:
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pgsql/contrib/xml2:
README.xml2 (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/README.xml2.diff?r1=1.7&r2=1.8)