Improve documentation note about Python 2.3 and cdecimal.

Explain exactly what fails (ie, function arguments of type numeric)
if you don't have it.

doc/src/sgml/installation.sgml

@@ -204,10 +204,10 @@ su - postgres
       installation with the header files and
       the <application>distutils</application> module.  The minimum
       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.
+      (To work with function arguments of type <type>numeric</>, a 2.3.x
+      installation must include the separately-available <filename>cdecimal</>
+      module; note 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]]>