sched_ext: Document task ownership state machine

sched_ext: Document task ownership state machine

The task ownership state machine in sched_ext is quite hard to follow
from the code alone. The interaction of ownership states, memory
ordering rules and cross-CPU "lock dancing" makes the overall model
subtle.

Extend the documentation next to scx_ops_state to provide a more
structured and self-contained description of the state transitions and
their synchronization rules.

The new reference should make the code easier to reason about and
maintain and can help future contributors understand the overall
task-ownership workflow.

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