doc: Remove buggy ICU collation from documentation

We have had multiple reports that point to the
'@colReorder=latn-digit' collation customization being buggy.  We have
reported this to ICU and are waiting for a fix.  In the meantime,
remove references to this from the documentation and replace it by
another reordering example.  Apparently, many users have been picking
up this example specifically from the documentation.

Author: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Discussion: https://www.postgresql.org/message-id/flat/153201618542.1404.3611626898935613264%40wrigleys.postgresql.org

doc/src/sgml/charset.sgml

@@ -791,11 +791,11 @@ CREATE COLLATION german (provider = libc, locale = 'de_DE');
      </varlistentry>
 
      <varlistentry>
-      <term><literal>CREATE COLLATION digitslast (provider = icu, locale = 'en-u-kr-latn-digit');</literal></term>
-      <term><literal>CREATE COLLATION digitslast (provider = icu, locale = 'en@colReorder=latn-digit');</literal></term>
+      <term><literal>CREATE COLLATION latinlast (provider = icu, locale = 'en-u-kr-grek-latn');</literal></term>
+      <term><literal>CREATE COLLATION latinlast (provider = icu, locale = 'en@colReorder=grek-latn');</literal></term>
       <listitem>
        <para>
-        Sort digits after Latin letters.  (The default is digits before letters.)
+        Sort Greek letters before Latin ones.  (The default is Latin before Greek.)
        </para>
       </listitem>
      </varlistentry>
@@ -812,8 +812,8 @@ CREATE COLLATION german (provider = libc, locale = 'de_DE');
      </varlistentry>
 
     <varlistentry>
-      <term><literal>CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-latn-digit');</literal></term>
-      <term><literal>CREATE COLLATION special (provider = icu, locale = 'en@colCaseFirst=upper;colReorder=latn-digit');</literal></term>
+      <term><literal>CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-grek-latn');</literal></term>
+      <term><literal>CREATE COLLATION special (provider = icu, locale = 'en@colCaseFirst=upper;colReorder=grek-latn');</literal></term>
       <listitem>
        <para>
         Combines both of the above options.