doc: add missing documentation

doc/doxygen/chapters/api/profiling.doxy

@@ -124,6 +124,11 @@ previous status is returned.
 Return the current profiling status or a negative value in case
 there was an error.
 
+\fn int starpu_profiling_init(void)
+\ingroup API_Profiling
+This function resets performance counters and enable profiling if the
+environment variable \ref STARPU_PROFILING is set to a positive value.
+
 \fn void starpu_profiling_set_id(int new_id)
 \ingroup API_Profiling
 This function sets the ID used for profiling trace filename. It

doc/doxygen/chapters/performance_feedback.doxy

@@ -35,7 +35,10 @@ call starpu_profiling_status_set() with the parameter
 is already enabled or not by calling starpu_profiling_status_get().
 Enabling monitoring also reinitialize all previously collected
 feedback. The environment variable \ref STARPU_PROFILING can also be
-set to <c>1</c> to achieve the same effect.
+set to <c>1</c> to achieve the same effect. The function
+starpu_profiling_init() can also be called during the execution to
+reinitialize performance counters and to start the profiling if the
+environment variable \ref STARPU_PROFILING is set to <c>1</c>.
 
 Likewise, performance monitoring is stopped by calling
 starpu_profiling_status_set() with the parameter