add the STARPU_SCHED_CTX attribut (for insert_task) to the doc

doc/chapters/api.texi

@@ -2326,7 +2326,7 @@ The arguments following the codelets can be of the following types:
 @item
 the specific values @code{STARPU_VALUE}, @code{STARPU_CALLBACK},
 @code{STARPU_CALLBACK_ARG}, @code{STARPU_CALLBACK_WITH_ARG},
-@code{STARPU_PRIORITY}, @code{STARPU_TAG}, @code{STARPU_FLOPS}, followed by the appropriated objects
+@code{STARPU_PRIORITY}, @code{STARPU_TAG}, @code{STARPU_FLOPS}, @code{STARPU_SCHED_CTX} followed by the appropriated objects
 as defined below.
 @end itemize
 
@@ -2379,6 +2379,10 @@ by an amount of floating point operations, as a double. The user may have to
 explicitly cast into double, otherwise parameter passing will not work.
 @end defmac
 
+@defmac STARPU_SCHED_CTX
+this macro is used when calling @code{starpu_insert_task}, and must be followed
+by the id of the scheduling context to which we want to submit the task.
+
 @deftypefun void starpu_codelet_pack_args ({void **}@var{arg_buffer}, {size_t *}@var{arg_buffer_size}, ...)
 Pack arguments of type @code{STARPU_VALUE} into a buffer which can be
 given to a codelet and later unpacked with the function