Document that Python 2.3 requires cdecimal module for full functionality.

This has been true for some time, but we were leaving users to discover it
the hard way.

Back-patch to 9.2.  It might've been true before that, but we were claiming
Python 2.2 compatibility before that, so I won't guess at the exact
requirements back then.

doc/src/sgml/installation.sgml

@@ -203,8 +203,12 @@ su - postgres
       language, you need a <productname>Python</productname>
       installation with the header files and
       the <application>distutils</application> module.  The minimum
-      required version is <productname>Python</productname>
-      2.3.  <productname>Python 3</productname> is supported if it's
+      required version is <productname>Python</productname> 2.3.
+      For full functionality, a 2.3.x installation must include the
+      separately-available <filename>cdecimal</> module; in particular
+      the <application>PL/Python</> regression tests will not pass if that
+      is missing.
+      <productname>Python 3</productname> is supported if it's
       version 3.1 or later; but see
       <![%standalone-include[the <application>PL/Python</> documentation]]>
       <![%standalone-ignore[<xref linkend="plpython-python23">]]>