grapheme_strlen : Document return null on failure (#5568)

* The grapheme_strlen function returns null on encoding failures

Co-authored-by: Christian Schneider <schneider@search.ch>

Author: chschneider

reference/intl/grapheme/grapheme-strlen.xml

@@ -36,9 +36,9 @@
 
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
-  <para>
-   The length of the string on success, &return.falseforfailure;. 
-  </para>
+  <simpara>
+   The length of the string on success, &null; or &false; on failure.
+  </simpara>
  </refsect1>
 
  <refsect1 role="examples">