pgsql: Improve cross-references between minor version release notes.

Improve cross-references between minor version release notes.

We have a practice of providing a "bread crumb" trail between the minor
versions where the migration section actually tells you to do something.
Historically that was just plain text, eg, "see the release notes for
9.2.4"; but if you're using a browser or PDF reader, it's a lot nicer
if it's a live hyperlink. So use "<xref>" instead. Any argument against
doing this vanished with the recent decommissioning of plain-text release
notes.

Vik Fearing

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/16b06c994f5d7d471f7284afdcd02c0a9ac93be1

Modified Files
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doc/src/sgml/release-7.4.sgml | 138 ++++++++++++++++-----------------
doc/src/sgml/release-8.0.sgml | 128 +++++++++++++++---------------
doc/src/sgml/release-8.1.sgml | 96 +++++++++++------------
doc/src/sgml/release-8.2.sgml | 34 ++++----
doc/src/sgml/release-8.3.sgml | 44 +++++------
doc/src/sgml/release-8.4.sgml | 34 ++++----
doc/src/sgml/release-9.0.sgml | 22 +++---
doc/src/sgml/release-old.sgml | 172 ++++++++++++++++++++---------------------
8 files changed, 334 insertions(+), 334 deletions(-)