Adding xreflable

You might know that while SGML's <link> requires you to specify link
text, <xref> can pull text from the "xreflabel" link site; our
doc/src/sgml/README.links file explains it:

<xref>
use to get chapter/section number from the title of the target
link, or xreflabel if defined at the target, or refentrytitle if target
is a refentry; has no close tag
http://www.oasis-open.org/docbook/documentation/reference/html/xref.html

linkend=
controls the target of the link/xref, required

endterm=
for <xref>, allows the text of the link/xref to be taken from a
different link target title

<link>
use to supply text for the link, only uses linkend, requires </link>
http://www.oasis-open.org/docbook/documentation/reference/html/link.html

I have noticed that our ref/*.sgml pages don't have such labels. I
would like to add them and use them for our release notes. Some of our
extensions need them too.

I think I need to backpatch this so that minor release notes can use
them too. OK? I have also noticed that the comment headings at the top
of our ref/*.sgml files are inconsistent and will fix those too.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
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