Doc: in each release branch, keep only that branch's own release notes.

Historically we've had each release branch include all prior branches'
notes, including minor-release changes, back to the beginning of the
project.  That's basically an O(N^2) proposition, and it was starting to
catch up with us: as of HEAD the back-branch release notes alone accounted
for nearly 30% of the documentation.  While there's certainly some value
in easy access to back-branch notes, this is getting out of hand.

Hence, switch over to the rule that each branch contains only its own
release notes.  So as to not make older notes too hard to find, each
branch will provide URLs for the immediately preceding branches'
release notes on the project website.

There might be value in providing aggregated notes across all branches
somewhere on the website, but that's a task for another day.

Discussion: https://postgr.es/m/cbd4aeb5-2d9c-8b84-e968-9e09393d4c83@postgresql.org

doc/src/sgml/filelist.sgml

@@ -169,19 +169,6 @@
 
 <!ENTITY release    SYSTEM "release.sgml">
 <!ENTITY release-9.6    SYSTEM "release-9.6.sgml">
-<!ENTITY release-9.5    SYSTEM "release-9.5.sgml">
-<!ENTITY release-9.4    SYSTEM "release-9.4.sgml">
-<!ENTITY release-9.3    SYSTEM "release-9.3.sgml">
-<!ENTITY release-9.2    SYSTEM "release-9.2.sgml">
-<!ENTITY release-9.1    SYSTEM "release-9.1.sgml">
-<!ENTITY release-9.0    SYSTEM "release-9.0.sgml">
-<!ENTITY release-8.4    SYSTEM "release-8.4.sgml">
-<!ENTITY release-8.3    SYSTEM "release-8.3.sgml">
-<!ENTITY release-8.2    SYSTEM "release-8.2.sgml">
-<!ENTITY release-8.1    SYSTEM "release-8.1.sgml">
-<!ENTITY release-8.0    SYSTEM "release-8.0.sgml">
-<!ENTITY release-7.4    SYSTEM "release-7.4.sgml">
-<!ENTITY release-old    SYSTEM "release-old.sgml">
 
 <!ENTITY acronyms   SYSTEM "acronyms.sgml">
 

doc/src/sgml/release.sgml

@@ -68,24 +68,81 @@ For new features, add links to the documentation sections.
   </para>
 
 <!--
-  To add a new major-release series, add an entry here and in filelist.sgml.
+  When beginning a new major-release series, create a new release-NN.sgml
+  file, removing the previous one, and change the &-reference here.
+  Don't forget to update filelist.sgml.  Also, add a link to the on-line
+  documentation for the newly forked-off branch in the list below.
 
-  The reason for splitting the release notes this way is so that appropriate
-  subsets can easily be copied into back branches.
+  The reason for keeping each branch's release notes in a differently-named
+  file is to reduce confusion when preparing minor-release updates.
+  All the active branches have to be edited concurrently when doing that.
 -->
+
 &release-9.6;
-&release-9.5;
-&release-9.4;
-&release-9.3;
-&release-9.2;
-&release-9.1;
-&release-9.0;
-&release-8.4;
-&release-8.3;
-&release-8.2;
-&release-8.1;
-&release-8.0;
-&release-7.4;
-&release-old;
+
+ <sect1 id="release-prior">
+  <title>Prior Releases</title>
+
+  <para>
+   Release notes for prior release branches can be found on the
+   <ulink url="https://www.postgresql.org/"><productname>PostgreSQL</productname></ulink>
+   web site.  At the time of release of version 9.6,
+   these were the supported prior release branches:
+
+   <itemizedlist>
+    <listitem>
+     <para>
+      <productname>PostgreSQL</productname> 9.5:
+      <ulink url="https://www.postgresql.org/docs/9.5/release.html">
+       <literal>https://www.postgresql.org/docs/9.5/release.html</literal>
+      </ulink>
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <productname>PostgreSQL</productname> 9.4:
+      <ulink url="https://www.postgresql.org/docs/9.4/release.html">
+       <literal>https://www.postgresql.org/docs/9.4/release.html</literal>
+      </ulink>
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <productname>PostgreSQL</productname> 9.3:
+      <ulink url="https://www.postgresql.org/docs/9.3/release.html">
+       <literal>https://www.postgresql.org/docs/9.3/release.html</literal>
+      </ulink>
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <productname>PostgreSQL</productname> 9.2:
+      <ulink url="https://www.postgresql.org/docs/9.2/release.html">
+       <literal>https://www.postgresql.org/docs/9.2/release.html</literal>
+      </ulink>
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <productname>PostgreSQL</productname> 9.1:
+      <ulink url="https://www.postgresql.org/docs/9.1/release.html">
+       <literal>https://www.postgresql.org/docs/9.1/release.html</literal>
+      </ulink>
+     </para>
+    </listitem>
+   </itemizedlist>
+  </para>
+
+  <para>
+   Release notes for older release branches can be found at
+   <ulink url="https://www.postgresql.org/docs/manuals/archive/">
+    <literal>https://www.postgresql.org/docs/manuals/archive/</literal>
+   </ulink>
+  </para>
+ </sect1>
 
 </appendix>