doc: Improve rendering of notes/cautions using XSL-FO

Center title and put a border around it, like the output that the DSSSL
version gave.

doc/src/sgml/stylesheet-fo.xsl

@@ -18,6 +18,20 @@
   <xsl:attribute name="wrap-option">wrap</xsl:attribute>
 </xsl:attribute-set>
 
+<xsl:attribute-set name="nongraphical.admonition.properties">
+  <xsl:attribute name="border-style">solid</xsl:attribute>
+  <xsl:attribute name="border-width">1pt</xsl:attribute>
+  <xsl:attribute name="border-color">black</xsl:attribute>
+  <xsl:attribute name="padding-start">12pt</xsl:attribute>
+  <xsl:attribute name="padding-end">12pt</xsl:attribute>
+  <xsl:attribute name="padding-top">6pt</xsl:attribute>
+  <xsl:attribute name="padding-bottom">6pt</xsl:attribute>
+</xsl:attribute-set>
+
+<xsl:attribute-set name="admonition.title.properties">
+  <xsl:attribute name="text-align">center</xsl:attribute>
+</xsl:attribute-set>
+
 <!-- Change display of some elements -->
 
 <xsl:template match="command">