Adjust comments about collate.linux.utf8 regression test.

This test should now work in any database with UTF8 encoding, regardless
of the database's default locale.  The former restriction was really
"doesn't work if default locale is C", and that was because of not handling
mbstowcs/wcstombs correctly.

doc/src/sgml/regress.sgml

@@ -254,7 +254,7 @@ gmake check EXTRA_TESTS=numeric_big
 gmake check EXTRA_TESTS=collate.linux.utf8 LANG=en_US.utf8
 </screen>
     The <literal>collate.linux.utf8</> test works only on Linux/glibc
-    platforms, and only when run in a locale that uses UTF-8 encoding.
+    platforms, and only when run in a database that uses UTF-8 encoding.
    </para>
   </sect2>
   </sect1>

src/test/regress/expected/collate.linux.utf8.out

 /*
  * This test is for Linux/glibc systems and assumes that a full set of
- * locales is installed.  It must be run in a UTF-8 locale.
+ * locales is installed.  It must be run in a database with UTF-8 encoding,
+ * because other encodings don't support all the characters used.
  */
 SET client_encoding TO UTF8;
 CREATE TABLE collate_test1 (

src/test/regress/sql/collate.linux.utf8.sql

 /*
  * This test is for Linux/glibc systems and assumes that a full set of
- * locales is installed.  It must be run in a UTF-8 locale.
+ * locales is installed.  It must be run in a database with UTF-8 encoding,
+ * because other encodings don't support all the characters used.
  */
 
 SET client_encoding TO UTF8;