doc: revert #3536. starpu_insert_task() returns an int

doc/starpu.texi

@@ -1277,7 +1277,7 @@ transfers, which are assumed to be completely overlapped.
 StarPU provides the wrapper function @code{starpu_insert_task} to ease
 the creation and submission of tasks.
 
-@deftypefun void starpu_insert_task (starpu_codelet *@var{cl}, ...)
+@deftypefun int starpu_insert_task (starpu_codelet *@var{cl}, ...)
 Create and submit a task corresponding to @var{cl} with the following
 arguments.  The argument list must be zero-terminated.