doc for the prolog callback

doc/doxygen/chapters/api/codelet_and_tasks.doxy

@@ -406,6 +406,21 @@ Optional field, the default value is <c>NULL</c>. This is the pointer
 passed to the callback function. This field is ignored if the field
 starpu_task::callback_func is set to <c>NULL</c>.
 
+\var starpu_task::prolog_func
+Optional field, the default value is <c>NULL</c>. This is a function
+pointer of prototype <c>void (*f)(void *)</c> which specifies a
+possible callback. 
+If this pointer is non-null, the callback function
+is executed on the host just before the task is pushed. Tasks which
+depend on it might already be executing. The callback is passed the
+value contained in the starpu_task::prolog_arg field. No callback is
+executed if the field is set to NULL.
+
+\var starpu_task::prolog_arg (optional) (default: NULL)
+Optional field, the default value is <c>NULL</c>. This is the pointer
+passed to the prolog function. This field is ignored if the field
+starpu_task::prolog_func is set to <c>NULL</c>.
+
 \var starpu_task::use_tag
 Optional field, the default value is 0. If set, this flag indicates
 that the task should be associated with the tag contained in the