Add a docs section for obsoleted and renamed functions and settings

The new appendix groups information on renamed or removed settings,
commands, etc into an out-of-the-way part of the docs.

The original id elements are retained in each subsection to ensure that
the same filenames are produced for HTML docs. This prevents /current/
links on the web from breaking, and allows users of the web docs
to follow links from old version pages to info on the changes in the
new version. Prior to this change, a link to /current/ for renamed
sections like the recovery.conf docs would just 404. Similarly if
someone searched for recovery.conf they would find the pg11 docs,
but there would be no /12/ or /current/ link, so they couldn't easily
find out that it was removed in pg12 or how to adapt.

Index entries are also added so that there's a breadcrumb trail for
users to follow when they know the old name, but not what we changed it
to. So a user who is trying to find out how to set standby_mode in
PostgreSQL 12+, or where pg_resetxlog went, now has more chance of
finding that information.

Craig Ringer and Stephen Frost
Reviewed-by: Euler Taveira
Discussion: https://postgr.es/m/CAGRY4nzPNOyYQ_1-pWYToUVqQ0ThqP5jdURnJMZPm539fdizOg%40mail.gmail.com
Backpatch-through: 10

doc/src/sgml/high-availability.sgml

@@ -613,9 +613,17 @@ protocol to make nodes agree on a serializable transactional order.
 
   </sect2>
 
-  <sect2 id="standby-server-operation">
+  <sect2 id="standby-server-operation" xreflabel="Standby Server Operation">
    <title>Standby Server Operation</title>
 
+   <para>
+    A server enters standby mode if a
+    <anchor id="file-standby-signal" xreflabel="standby.signal"/>
+    <filename>standby.signal</filename>
+    <indexterm><primary><filename>standby.signal</filename></primary></indexterm>
+    file exists in the data directory when the server is started.
+   </para>
+
    <para>
     In standby mode, the server continuously applies WAL received from the
     primary server. The standby server can read WAL from a WAL archive
@@ -689,7 +697,8 @@ protocol to make nodes agree on a serializable transactional order.
    <para>
     To set up the standby server, restore the base backup taken from primary
     server (see <xref linkend="backup-pitr-recovery"/>). Create a file
-    <filename>standby.signal</filename> in the standby's cluster data
+    <link linkend="file-standby-signal"><filename>standby.signal</filename></link><indexterm><primary>standby.signal</primary></indexterm>
+    in the standby's cluster data
     directory. Set <xref linkend="guc-restore-command"/> to a simple command to copy files from
     the WAL archive. If you plan to have multiple standby servers for high
     availability purposes, make sure that <varname>recovery_target_timeline</varname> is set to
@@ -1936,7 +1945,8 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
 
    <para>
     If <varname>hot_standby</varname> is <literal>on</literal> in <filename>postgresql.conf</filename>
-    (the default value) and there is a <filename>standby.signal</filename>
+    (the default value) and there is a
+    <link linkend="file-standby-signal"><filename>standby.signal</filename></link><indexterm><primary>standby.signal</primary><secondary>for hot standby</secondary></indexterm>
     file present, the server will run in Hot Standby mode.
     However, it may take some time for Hot Standby connections to be allowed,
     because the server will not accept connections until it has completed