sched/deadline: document new sched_getattr() feature for retrieving current parameter...

sched/deadline: document new sched_getattr() feature for retrieving current parameters for DEADLINE tasks

Document in Documentation/sched/sched-deadline.rst the new capability of
sched_getattr() to retrieve, for DEADLINE tasks, the runtime left and absolute
deadline (setting the flags syscall parameter to 1), in addition to the static
parameters (obtained with flags=0).

Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260304102843.1373905-2-tommaso.cucinotta@santannapisa.it>