Qualification about callables #2632

doc/Type/Hash.pod6

@@ -71,7 +71,8 @@ values using the given C<mapper>, optionally altering the values using the
 C<:&as> L«C<Callable>|/type/Callable». The C<@list> cannot be lazy.
 
 The mapper can be a L«C<Callable>|/type/Callable» that takes a single argument,
-an L«C<Associative>|/type/Associative», or an L«C<Iterable>|/type/Iterable».
+an L«C<Associative>|/type/Associative», or an L«C<Iterable>|/type/Iterable»;
+this C<Callable> is guaranteed to be called only once per item.
 With L«C<Associative>|/type/Associative» and an L«C<Iterable>|/type/Iterable»
 mappers, the values in the C<@list> represent the key and index of the mapper's
 value respectively. A L«C<Callable>|/type/Callable» mapper will be executed