signal: Add proper comment about the preempt-disable in ptrace_stop().

Commit 53da1d94

 ("fix ptrace slowness") added a preempt-disable section
between read_unlock() and the following schedule() invocation without
explaining why it is needed.

Replace the comment with an explanation why this is needed. Clarify that
it is needed for correctness but for performance reasons.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/20230406205713.1843072-2-bigeasy@linutronix.de