document value returned by push_task

doc/doxygen/chapters/api/modularized_scheduler.doxy

@@ -69,6 +69,9 @@ like <c>component->push_task(component,task)</c>
      recursive call on a child or store the task in the component,
      then it will be returned by a further pull_task call.
      the caller must ensure that component is able to execute task.
+     This method must either return 0 if it the task was properly stored or
+     passed over to a child component, or return a value different from 0 if the
+     task could not be consumed (e.g. the queue is full).
 \var struct starpu_task * (*starpu_sched_component::pull_task)(struct starpu_sched_component *)
      pop a task from the scheduler module. this function is called by workers to get a task from their
      parents. this function should first return a locally stored task
@@ -79,13 +82,13 @@ like <c>component->push_task(component,task)</c>
      This function is called by a component which implements a queue,
      allowing it to signify to its parents that an empty slot is
      available in its queue. The basic implementation of this function
-     is a recursive call to its parents, the user have to specify a
+     is a recursive call to its parents, the user has to specify a
      personally-made function to catch those calls.
 \var void (*starpu_sched_component::can_pull)(struct starpu_sched_component *component)
      This function allow a component to wake up a worker. It is
      currently called by component which implements a queue, to
      signify to its children that a task have been pushed in its local
-     queue, and is available to been popped by a worker, for example.
+     queue, and is available to be popped by a worker, for example.
      The basic implementation of this function is a recursive call to
      its children, until at least one worker have been woken up.