doc: Add support for xref to command and function elements

Discussion: https://www.postgresql.org/message-id/517abe28-8a93-5b7a-ff40-b1fd61d33b26%402ndquadrant.com

doc/src/sgml/stylesheet-common.xsl

@@ -86,4 +86,15 @@
   <xsl:text>?</xsl:text>
 </xsl:template>
 
+
+<!-- Support for generating xref link text to additional elements -->
+
+<xsl:template match="command" mode="xref-to">
+  <xsl:apply-templates select="." mode="xref"/>
+</xsl:template>
+
+<xsl:template match="function" mode="xref-to">
+  <xsl:apply-templates select="." mode="xref"/>
+</xsl:template>
+
 </xsl:stylesheet>