doc: update documentation build instructions

Also add an item in the release notes

doc/appendix-release-notes.sgml

@@ -78,6 +78,25 @@
       </para>
     </sect2>
 
+   <sect2>
+      <title>Other</title>
+      <para>
+        <itemizedlist>
+          <listitem>
+            <para>
+              The &repmgr; documentation has been converted to DocBook XML format,
+              as currently used by the main PostgreSQL project.
+              This means it can now be built against any &repmgr; version
+              (previously it was not possible to build the documentation against
+              PostgreSQL 10 or later).
+            </para>
+            <para>
+              For further details see: <xref linkend="installation-build-repmgr-docs"/>
+            </para>
+          </listitem>
+        </itemizedlist>
+      </para>
+   </sect2>
   </sect1>
 
   <sect1 id="release-4.3.1">

doc/install-source.sgml

@@ -237,7 +237,7 @@ deb-src http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main</programlisti
 
 
 
- <sect2 id="installation-build-repmgr-docs">
+ <sect2 id="installation-build-repmgr-docs" xreflabel="Building repmgr documentation">
    <title>Building &repmgr; documentation</title>
    <para>
     The &repmgr; documentation is (like the main PostgreSQL project)
@@ -254,7 +254,8 @@ deb-src http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main</programlisti
    </para>
 
    <para>
-     To build the documentation as a single HTML file, execute:
+     To build the documentation as a single HTML file, after configuring and building
+     the main &repmgr; source as described above, execute:
    <programlisting>
     cd doc/ &amp;&amp; make repmgr.html</programlisting>
    </para>