doc PG relnotes: add paragraph explaining the section symbol

And suppress the symbol in print mode, where the section symbol does not
appear.

Discussion: https://postgr.es/m/ZuobILbmGGetxEg5@momjian.us

Backpatch-through: 12

doc/src/sgml/release.sgml

@@ -69,6 +69,15 @@ For new features, add links to the documentation sections.
    review, so each item is truly a community effort.
   </para>
 
+  <para id="release-no-print-output">
+   Section markers (&sect;) in the release notes link to <ulink
+   url="https://git.postgresql.org/gitweb/?p=postgresql.git"><application>gitweb</application></ulink>
+   pages which show the primary <application>git</application> commit
+   messages and source tree changes responsible for the release note item.
+   There might be additional <application>git</application> commits which
+   are not shown.
+  </para>
+
 <!--
   When beginning a new major-release series, create a new release-NN.sgml
   file, removing the previous one, and change the &-reference here.

doc/src/sgml/stylesheet-fo.xsl

@@ -20,6 +20,11 @@
   <!-- Do nothing for ulink to avoid footnotes -->
 </xsl:template>
 
+<!-- Also suppress the description of the section markers in print mode -->
+<xsl:template match="appendix[@id='release']//para[@id='release-no-print-output']">
+  <!-- Output nothing for this para -->
+</xsl:template>
+
 <xsl:param name="use.extensions" select="1"></xsl:param>
 <xsl:param name="variablelist.as.blocks" select="1"></xsl:param>
 <xsl:param name="orderedlist.label.width">1.5em</xsl:param>