Fix GH-1178: array_flip allegedly can return null

Author: cmb69

reference/array/functions/array-flip.xml

@@ -48,7 +48,7 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <para>
-   Returns the flipped array on success and &null; on failure.
+   Returns the flipped array.
   </para>
  </refsect1>