doc: Add more ICU rules examples

In particular, add an example EBCDIC collation.

Author: Daniel Verite <daniel@manitou-mail.org>
Discussion: https://www.postgresql.org/message-id/flat/35cc1684-e516-4a01-a256-351632d47066@manitou-mail.org

doc/src/sgml/charset.sgml

@@ -1481,7 +1481,7 @@ SELECT 'x-y' = 'x_y' COLLATE level4; -- false
    </sect3>
 
    <sect3 id="icu-locale-examples">
-    <title>Examples</title>
+    <title>Collation Settings Examples</title>
 
      <variablelist>
       <varlistentry id="collation-managing-create-icu-de-u-co-phonebk-x-icu">
@@ -1530,6 +1530,62 @@ SELECT 'x-y' = 'x_y' COLLATE level4; -- false
      </variablelist>
    </sect3>
 
+   <sect3 id="icu-tailoring-rules">
+    <title>ICU Tailoring Rules</title>
+
+    <para>
+     If the options provided by the collation settings shown above are not
+     sufficient, the order of collation elements can be changed with tailoring
+     rules, whose syntax is detailed at <ulink
+     url="https://unicode-org.github.io/icu/userguide/collation/customization/"></ulink>.
+    </para>
+
+    <para>
+     This small example creates a collation based on the root locale with a
+     tailoring rule:
+<programlisting>
+<![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.)
+    </para>
+
+    <para>
+     Here is a more complex example.  The following statement sets up a
+     collation named <literal>ebcdic</literal> with rules to sort US-ASCII
+     characters in the order of the EBCDIC encoding.
+
+<programlisting>
+<![CDATA[CREATE COLLATION ebcdic (provider = icu, locale = 'und',
+rules = $$
+& ' ' < '.' < '<' < '(' < '+' < \|
+< '&' < '!' < '$' < '*' < ')' < ';'
+< '-' < '/' < ',' < '%' < '_' < '>' < '?'
+< '`' < ':' < '#' < '@' < \' < '=' < '"'
+<*a-r < '~' <*s-z < '^' < '[' < ']'
+< '{' <*A-I < '}' <*J-R < '\' <*S-Z <*0-9
+$$);]]>
+
+SELECT c
+FROM (VALUES ('a'), ('b'), ('A'), ('B'), ('1'), ('2'), ('!'), ('^')) AS x(c)
+ORDER BY c COLLATE ebcdic;
+ c
+---
+ !
+ a
+ b
+ ^
+ A
+ B
+ 1
+ 2
+</programlisting>
+    </para>
+   </sect3>
+
    <sect3 id="icu-external-references">
     <title>External References for ICU</title>
 

doc/src/sgml/ref/create_collation.sgml

@@ -165,9 +165,8 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Specifies additional collation rules to customize the behavior of the
-       collation.  This is supported for ICU only.  See <ulink
-       url="https://unicode-org.github.io/icu/userguide/collation/customization/"/>
-       for details on the syntax.
+       collation.  This is supported for ICU only.  See <xref
+       linkend="icu-tailoring-rules"/> for details.
       </para>
      </listitem>
     </varlistentry>
@@ -257,12 +256,8 @@ CREATE COLLATION german_phonebook (provider = icu, locale = 'de-u-co-phonebk');
 <programlisting>
 <![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.
+   See <xref linkend="icu-tailoring-rules"/> for further details and examples
+   on the rules syntax.
   </para>
 
   <para>

doc/src/sgml/ref/create_database.sgml

@@ -232,9 +232,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
        <para>
         Specifies additional collation rules to customize the behavior of the
         default collation of this database.  This is supported for ICU only.
-        See <ulink
-        url="https://unicode-org.github.io/icu/userguide/collation/customization/"/>
-        for details on the syntax.
+        See <xref linkend="icu-tailoring-rules"/> for details.
        </para>
       </listitem>
      </varlistentry>