doc: posix: sched functions will fail with ENOSYS

Mark `sched_setparam()` , `sched_setscheduler()`
and `sched_rr_get_interval()`
have implemented but will fail with ``ENOSYS``
Were missing on the documentation before.

signed-off-by: Gaetan Perrot <gaetanperrotpro@gmail.com>
1 file changed