fix prologue documentation

ChangeLog

@@ -48,8 +48,8 @@ New features:
     to switch easily between differents versions of StarPU having
     different performance model formats.
   * Tasks can now define a optional prologue callback which is executed
-    on the host just before the task is pushed. Tasks which depend on
-    the task might already be executing.
+    on the host when the task becomes ready for execution, before getting
+    scheduled.
 
 Small features:
   * Add cl_arg_free field to enable automatic free(cl_arg) on task

doc/doxygen/chapters/api/codelet_and_tasks.doxy

@@ -406,20 +406,20 @@ 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
+\var starpu_task::prologue_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
+is executed on the host when the task becomes ready for execution,
+before getting scheduled. The callback is passed the
+value contained in the starpu_task::prologue_arg field. No callback is
 executed if the field is set to NULL.
 
-\var starpu_task::prolog_arg (optional) (default: NULL)
+\var starpu_task::prologue_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>.
+passed to the prologue function. This field is ignored if the field
+starpu_task::prologue_func is set to <c>NULL</c>.
 
 \var starpu_task::use_tag
 Optional field, the default value is 0. If set, this flag indicates