'PrefixSearchBackend': Override the title prefix search used for OpenSearch and
AJAX search suggestions. Put results into &$results outparam and return false.
-$ns : int namespace key to search in
+$ns : array of int namespace keys to search in
$search : search term (not guaranteed to be conveniently normalized)
$limit : maximum number of results to return
&$results : out param: array of page names (strings)