Doc: revise formatting of function/operator tables.

The table layout ideas proposed in commit e894c6183 were not as widely
popular as I'd hoped.  After discussion, we've settled on a layout
that's effectively a single-column table with cell contents much like a
<varlistentry> description of the function or operator; though we're not
actually using <varlistentry>, because it'd add way too much vertical
space.  Instead the effect is accomplished using line-break processing
instructions to separate the description and example(s), plus CSS or FO
customizations to produce indentation of all but the first line in each
cell.  While technically this is a bit grotty, it does have the
advantage that we won't need to write nearly as much boilerplate markup.

This patch updates tables 9.30, 9.31, and 9.33 (which were touched by
the previous patch) to the revised style, and additionally converts
table 9.10.  A lot of work still remains to do, but hopefully it won't
be too controversial.

Thanks to Andrew Dunstan, Pierre Giraud, Robert Haas, Alvaro Herrera,
David Johnston, Jonathan Katz, Isaac Morland for valuable ideas.

Discussion: https://postgr.es/m/8691.1586798003@sss.pgh.pa.us

doc/src/sgml/stylesheet-common.xsl

@@ -103,4 +103,11 @@
   <xsl:apply-templates select="." mode="xref"/>
 </xsl:template>
 
+
+<!-- Support for explicit line breaks <?br?> within table cells -->
+
+<xsl:template match="processing-instruction('br')">
+  <br/>
+</xsl:template>
+
 </xsl:stylesheet>