sched_ext: Improve SCX_KF_DISPATCH comment

The comment for SCX_KF_DISPATCH was incomplete and didn't explain that
ops.dispatch() may temporarily release the rq lock, allowing ENQUEUE and
SELECT_CPU operations to be nested inside DISPATCH contexts.

Update the comment to clarify this nesting behavior and provide better
context for when these operations can occur within dispatch.

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