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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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/42ec6a832348f168eb94443d168bdb2f62fe1794

Modified Files
--------------
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-old.sgml |  288 ++++++++++++++++++++---------------------
7 files changed, 381 insertions(+), 381 deletions(-)