doc: Add deadline scheduling information

to be the current thread. When multiple ready threads of the same priority

exist, the scheduler chooses the one that has been waiting longest.

A thread's relative priority is primarily determined by its static priority.

However, when both earliest-deadline-first scheduling is enabled

(:kconfig:`CONFIG_SCHED_DEADLINE`) and a choice of threads have equal

static priority, then the thread with the earlier deadline is considered

to have the higher priority. Thus, when earliest-deadline-first scheduling is

enabled, two threads are only considered to have the same priority when both

their static priorities and deadlines are equal. The routine

:c:func:`k_thread_deadline_set` is used to set a thread's deadline.

.. note::

    Execution of ISRs takes precedence over thread execution,

    so the execution of the current thread may be replaced by an ISR