perf: Add comment to enum perf_event_state

Better describe the event states.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Link: https://lkml.kernel.org/r/20250604135801.GK38114@noisy.programming.kicks-ass.net