Doc: fix "Unresolved ID reference" warnings, clean up man page cross-refs.

Use xreflabel attributes instead of endterm attributes to control the
appearance of links to subsections of SQL command reference pages.
This is simpler, it matches what we do elsewhere (e.g. for GUC variables),
and it doesn't draw "Unresolved ID reference" warnings from the PDF
toolchain.

Fix some places where the text was absolutely dependent on an <xref>
rendering exactly so, by using a <link> around the required text
instead.  At least one of those spots had already been turned into
bad grammar by subsequent changes, and the whole idea is just too
fragile for my taste.  <xref> does NOT have fixed output, don't write
as if it does.

Consistently include a page-level link in cross-man-page references,
because otherwise they are useless/nonsensical in man-page output.
Likewise, be consistent about mentioning "below" or "above" in same-page
references; we were doing that in about 90% of the cases, but now it's
100%.

Also get rid of another nonfunctional-in-PDF idea, of making
cross-references to functions by sticking ID tags on <row> constructs.
We can put the IDs on <indexterm>s instead --- which is probably not any
more sensible in abstract terms, but it works where the other doesn't.
(There is talk of attaching cross-reference IDs to most or all of
the docs' function descriptions, but for now I just fixed the two
that exist.)

Discussion: https://postgr.es/m/14480.1589154358@sss.pgh.pa.us

doc/src/sgml/ref/create_aggregate.sgml

@@ -434,8 +434,8 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
       This option specifies whether the final function is a pure function
       that does not modify its arguments.  <literal>READ_ONLY</literal> indicates
       it does not; the other two values indicate that it may change the
-      transition state value.  See <xref linkend="sql-createaggregate-notes"
-      endterm="sql-createaggregate-notes-title"/> below for more detail.  The
+      transition state value.  See <xref linkend="sql-createaggregate-notes"/>
+      below for more detail.  The
       default is <literal>READ_ONLY</literal>, except for ordered-set aggregates,
       for which the default is <literal>READ_WRITE</literal>.
      </para>
@@ -664,8 +664,8 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
   </para>
  </refsect1>
 
- <refsect1 id="sql-createaggregate-notes">
-  <title id="sql-createaggregate-notes-title">Notes</title>
+ <refsect1 id="sql-createaggregate-notes" xreflabel="Notes">
+  <title>Notes</title>
 
    <para>
     In parameters that specify support function names, you can write