doc: Better example for custom ICU rules

Use a more practical example, and also add some explanation.

Reported-by: Jeff Davis <pgsql@j-davis.com>

Author: petere

doc/src/sgml/ref/create_collation.sgml

@@ -243,11 +243,17 @@ CREATE COLLATION german_phonebook (provider = icu, locale = 'de-u-co-phonebk');
   </para>
 
   <para>
-   To create a collation using the ICU provider, based on the English ICU
-   locale, with custom rules:
+   To create a collation using the ICU provider, based on the root ICU locale,
+   with custom rules:
 <programlisting>
-<![CDATA[CREATE COLLATION en_custom (provider = icu, locale = 'en', rules = '&a < g');]]>
+<![CDATA[CREATE COLLATION custom (provider = icu, locale = 'und', rules = '&V << w <<< W');]]>
 </programlisting>
+   With this rule, the letter <quote>W</quote> is sorted after
+   <quote>V</quote>, but is treated as a secondary difference similar to an
+   accent.  Rules like this are contained in the locale definitions of some
+   languages.  (Of course, if a locale definition already contains the desired
+   rules, then they don't need to be specified again explicitly.)  See the ICU
+   documentation for further details and examples on the rules syntax.
   </para>
 
   <para>