docs(jest): improve docs about fake timers

docs/jest.jade

@@ -42,11 +42,17 @@ block content
  ## Timer Mocks
 
  Absolutely do **not** use [timer mocks](https://jestjs.io/docs/en/timer-mocks.html)
- when testing Mongoose apps. Mongoose devs have already refactored out code
+ when testing Mongoose apps. Fake timers stub out global functions like
+ `setTimeout()` and `setInterval()`, which causes problems when an underlying
+ library uses these functions. The MongoDB Node.js driver uses these functions
+ for deferring work until the next tick of the event loop and for monitoring
+ connections to the MongoDB server.
+
+ Mongoose devs have already refactored out code
  to [avoid using `setImmediate()`](https:/Automattic/mongoose/issues/6074)
  to defer work to the next tick of the event loop, but we can't reasonably
  ensure that every library Mongoose depends on doesn't use `setImmediate()`.
- 
+
  Mongoose uses `nextTick()`, which [Jest's underlying dependency explicitly doesn't stub by default](https://sinonjs.org/releases/latest/fake-timers/).
  But, `process.nextTick()` isn't the same as `setImmediate()` because of
  [microtasks vs macrotasks](https://abc.danch.me/microtasks-macrotasks-more-on-the-event-loop-881557d7af6f#2261),