sched_ext: Documentation: Clarify time slice handling in task lifecycle

sched_ext: Documentation: Clarify time slice handling in task lifecycle

It is not always obvious how a task's time slice can be refilled, either
explicitly from ops.dispatch() or automatically by the sched_ext core,
to skip subsequent ops.enqueue() and ops.dispatch() calls. This
typically happens when the task is the only one running on a CPU.

To make this behavior easier to understand, update the task lifecycle
diagram to explicitly document how time slice handling works in such
cases.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>