doc: Change xref style to number only

Change the style of links generated by xrefs to section number only, as
it was with DSSSL, instead of number and title, as is the default of the
XSLT stylesheets.

Our documentation is mostly written expecting the old style, so keep
that for the time being, per discussion.

Author: petere

doc/src/sgml/stylesheet-common.xsl

@@ -39,6 +39,7 @@
 <xsl:param name="punct.honorific" select="''"></xsl:param>
 <xsl:param name="variablelist.term.break.after">1</xsl:param>
 <xsl:param name="variablelist.term.separator"></xsl:param>
+<xsl:param name="xref.with.number.and.title" select="0"></xsl:param>
 
 
 <!-- Change display of some elements -->