add a warning for callback and array with less than 2 items

Author: alexislefebvre

language-snippets.ent

@@ -739,6 +739,11 @@ use.</simpara></note>'>
   <literal>0.99</literal> and <literal>0.1</literal> will both be cast to an
   integer value of <literal>0</literal>, which will compare such values as equal.
  </para>
+ <para>
+  The callback is not called if the input has less than 2 items,
+  an error (e.g. accessing an invalid key of an array) or an exception
+  will not be reported, unless you test it with 2 items or more.
+ </para>
 </caution>'>
 
 <!ENTITY sort.callback.description.presort '<caution xmlns="http://docbook.org/ns/docbook">