better doc for setlocale (#4770)

Author: remicollet

reference/strings/functions/setlocale.xml

@@ -97,12 +97,12 @@
      <listitem>
       <para>
        If <parameter>locales</parameter> is the empty string
-       <literal>""</literal>, the locale names will be set from the
+       <literal>""</literal> or is &null;, the locale names will be set from the
        values of environment variables with the same names as the above
        categories, or from "LANG".
       </para>
       <para>
-       If <parameter>locales</parameter> is &null;,
+       If <parameter>locales</parameter> is <literal>"0"</literal>,
        the locale setting is not affected, only the current setting is returned.
       </para>
       <para>