This reverts commit 3d4fa227bc.
Per discussion and buildfarm, this depends on APIs that seem to not
be available on at least one platform (NetBSD). Should be certainly
possible to rework to be optional on that platform if necessary but bit
late for that at this point.
Discussion: https://postgr.es/m/3286097.1680922218@sss.pgh.pa.us
<!-- doc/src/sgml/README.links -->
Linking within DocBook documents can be confusing, so here is a summary:
Intra-document Linking
----------------------
<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
can be embedded inside of <command>, unlike <xref>
External Linking
----------------
<ulink>
like <link>, but uses a URL (not a document target); requires
</ulink>; if no text is specified, the URL appears as the link
text
http://www.oasis-open.org/docbook/documentation/reference/html/ulink.html
url=
used by <ulink> to specify the URL, required
Guidelines
----------
- For an internal link, if you want to supply text, use <link>, else
<xref>.
- Specific nouns like GUC variables, SQL commands, and contrib modules
usually have xreflabels.
- For an external link, use <ulink>, with or without link text.
- xreflabels added to tags prevent the chapter/section for id's from being
referenced; only the xreflabel is accessible. Therefore, use xreflabels
only when linking is common, and chapter/section information is unneeded.
3d03b24c35