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
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/2d77f16c1d8d942e0391d331dc62f3435a995d20

Modified Files
--------------
doc/src/sgml/release-7.4.sgml |   66 +++++++++++++++++++-------------------
doc/src/sgml/release-8.0.sgml |   70 ++++++++++++++++++++---------------------
doc/src/sgml/release-8.1.sgml |   42 ++++++++++++-------------
doc/src/sgml/release-8.2.sgml |   32 +++++++++----------
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-9.1.sgml |   18 +++++------
doc/src/sgml/release-9.2.sgml |   10 +++---
doc/src/sgml/release-9.3.sgml |    2 +-
doc/src/sgml/release-old.sgml |   44 +++++++++++++-------------
11 files changed, 192 insertions(+), 192 deletions(-)