Doc: render π more nicely in PDF output.

We need to select symbol font explicitly, or it comes out misaligned.

Alexander Lakhin, Tom Lane

Discussion: https://postgr.es/m/10598.1587928415@sss.pgh.pa.us

doc/src/sgml/func.sgml

@@ -1365,7 +1365,7 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
         <function>pi</function> (  )
         <returnvalue>double precision</returnvalue>
        <?br?>
-        Approximate value of &pi;
+        Approximate value of <phrase role="symbol_font">&pi;</phrase>
        <?br?>
         <literal>pi()</literal>
         <returnvalue>3.141592653589793</returnvalue>

doc/src/sgml/ref/pgbench.sgml

@@ -1528,7 +1528,7 @@ SELECT 4 AS four \; SELECT 5 AS five \aset
       <row>
        <entry><literal><function>pi()</function></literal></entry>
        <entry>double</entry>
-       <entry>value of the constant PI</entry>
+       <entry>value of the constant <phrase role="symbol_font">&pi;</phrase></entry>
        <entry><literal>pi()</literal></entry>
        <entry><literal>3.14159265358979323846</literal></entry>
       </row>

doc/src/sgml/stylesheet-fo.xsl

@@ -101,6 +101,11 @@
   <fo:block/>
 </xsl:template>
 
+<!-- FOP needs us to be explicit about use of symbol font in some cases -->
+<xsl:template match="phrase[@role='symbol_font']">
+  <fo:inline font-family="{$symbol.font.family}"><xsl:value-of select="."/></fo:inline>
+</xsl:template>
+
 <!-- bug fix from <https://sourceforge.net/p/docbook/bugs/1360/#831b> -->
 
 <xsl:template match="varlistentry/term" mode="xref-to">