Document the minimum required Python version.

It turns out that Python 2.2 is the oldest version that PL/Python compiles
with, apparently related to the introduction of iterators.  Might as well
document this.

doc/src/sgml/installation.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.326 2009/07/13 01:51:56 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.327 2009/08/12 16:32:35 petere Exp $ -->
 
 <chapter id="installation">
  <title><![%standalone-include[<productname>PostgreSQL</>]]>
@@ -196,10 +196,7 @@ su - postgres
       To build the <application>PL/Python</> server programming
       language, you need a <productname>Python</productname>
       installation with the header files and the <application>distutils</application> module.
-      The <application>distutils</application> module is included by default with
-      <productname>Python</productname> 1.6 and later; users of
-      earlier versions of <productname>Python</productname> will need
-      to install it.
+      The minimum required version is <productname>Python</productname> 2.2.
      </para>
 
      <para>