https://redd.one/blog/debounce-vs-throttle
用动画的形式给出了差别的描述。
文字上来说， debounce
Discard emitted values that take less than the specified time, based on selector function, between output.
而 throttle
Emit value on the leading edge of an interval, but suppress new values until durationSelector has completed.
所以对某个时间段参数， 用throttle会保证一次发生（不会多）， 而debounce不能保证发生。
