Migration section added

I have added the attached text to the migration section.  It outlines
some of the areas that typically change between major releases.  I am
hopeful it will help people in testing migrations.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://postgres.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: doc/src/sgml/backup.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/backup.sgml,v
retrieving revision 2.113
diff -c -c -r2.113 backup.sgml
*** doc/src/sgml/backup.sgml    23 Jan 2008 20:21:37 -0000    2.113
--- doc/src/sgml/backup.sgml    13 Feb 2008 22:42:41 -0000
***************
*** 1910,1922 ****
    </para>

    <para>
-    In practice you probably want to test your client
-    applications on the new setup before switching over completely.
-    This is another reason for setting up concurrent installations
-    of old and new versions.
-   </para>
-
-   <para>
     If you cannot or do not want to run two servers in parallel, you can
     do the backup step before installing the new version, bring down
     the server, move the old version out of the way, install the new
--- 1910,1915 ----
***************
*** 1955,1959 ****
--- 1948,2015 ----
      moving an installation like this will not work.)
     </para>
    </note>
+
+   <para>
+    In practice you probably want to test your client applications on the
+    new version before switching over completely.  This is another reason
+    for setting up concurrent installations of old and new versions.  When
+    testing a <productname>PostgreSQL</> major upgrade, consider the
+    following categories of possible changes:
+   </para>
+
+   <variablelist>
+
+    <varlistentry>
+     <term>Administration</term>
+     <listitem>
+      <para>
+       The capabilities available for administrators to monitor and control
+       the server often change and improve in each major release.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term>SQL</term>
+     <listitem>
+      <para>
+       Typically this includes new SQL command capabilities and not changes
+       in behavior, unless specifically mentioned in the release notes.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term>Library API</term>
+     <listitem>
+      <para>
+       Typically libraries like <application>libpq</> only add new
+       functionality, again unless mentioned in the release notes.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term>System Catalogs</term>
+     <listitem>
+      <para>
+       System catalog changes usually only affect database management tools.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term>Server C-language API</term>
+     <listitem>
+      <para>
+       This involved changes in the backend function API, which is written
+       in the C programming language.  Such changes effect code that
+       references backend functions deep inside the server.
+      </para>
+     </listitem>
+    </varlistentry>
+
+   </variablelist>
+
   </sect1>
  </chapter>