Expand collation documentation

Document better how to create custom collations and what locale strings
ICU accepts.  Explain the ICU examples in more detail.  Also update the
text on the CREATE COLLATION reference page a bit to take ICU more into
account.

doc/src/sgml/ref/create_collation.sgml

@@ -93,10 +93,7 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Use the specified operating system locale for
-       the <symbol>LC_COLLATE</symbol> locale category.  The locale
-       must be applicable to the current database encoding.
-       (See <xref linkend="sql-createdatabase"> for the precise
-       rules.)
+       the <symbol>LC_COLLATE</symbol> locale category.
       </para>
      </listitem>
     </varlistentry>
@@ -107,10 +104,7 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Use the specified operating system locale for
-       the <symbol>LC_CTYPE</symbol> locale category.  The locale
-       must be applicable to the current database encoding.
-       (See <xref linkend="sql-createdatabase"> for the precise
-       rules.)
+       the <symbol>LC_CTYPE</symbol> locale category.
       </para>
      </listitem>
     </varlistentry>
@@ -173,8 +167,13 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
   </para>
 
   <para>
-   See <xref linkend="collation"> for more information about collation
-   support in PostgreSQL.
+   See <xref linkend="collation-create"> for more information on how to create collations.
+  </para>
+
+  <para>
+   When using the <literal>libc</literal> collation provider, the locale must
+   be applicable to the current database encoding.
+   See <xref linkend="sql-createdatabase"> for the precise rules.
   </para>
  </refsect1>
 
@@ -186,7 +185,14 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
    <literal>fr_FR.utf8</literal>
    (assuming the current database encoding is <literal>UTF8</literal>):
 <programlisting>
-CREATE COLLATION french (LOCALE = 'fr_FR.utf8');
+CREATE COLLATION french (locale = 'fr_FR.utf8');
+</programlisting>
+  </para>
+
+  <para>
+   To create a collation using the ICU provider using German phone book sort order:
+<programlisting>
+CREATE COLLATION german_phonebook (provider = icu, locale = 'de-u-co-phonebk');
 </programlisting>
   </para>