Doc: update sections 9.1-9.3 for new function table layout.

I took the opportunity to do some copy-editing in this area as well,
and to add some new material such as a note about BETWEEN's syntactical
peculiarities.

Of note is that quite a few of the examples of transcendental functions
needed to be updated, because the displayed output no longer matched
what you get on a modern server.  I believe some of these cases are
side-effects of the new Ryu algorithm in float8out.  Others appear to be
because the examples predate the addition of type numeric, and were
expecting that float8 calculations would be done although the given
syntax would actually lead to calling the numeric function nowadays.

doc/src/sgml/stylesheet-common.xsl

@@ -50,10 +50,8 @@
 </xsl:template>
 
 <!-- Render <returnvalue> with a right arrow then the type name -->
-<xsl:template match="returnvalue">
-  &#x2192;
-  <xsl:call-template name="inline.monoseq"/>
-</xsl:template>
+<!-- Avoid adding unnecessary white space in this template! -->
+<xsl:template match="returnvalue">&#x2192; <xsl:call-template name="inline.monoseq"/></xsl:template>
 
 <xsl:template match="structfield">
   <xsl:call-template name="inline.monoseq"/>