Doc fixes

Fixes #1701. Thanks baudehlo.

doc/api/timers.markdown

@@ -6,6 +6,11 @@ To schedule execution of a one-time `callback` after `delay` milliseconds. Retur
 `timeoutId` for possible use with `clearTimeout()`. Optionally you can
 also pass arguments to the callback.
 
+It is important to note that your callback will probably not be called in exactly
+`delay` milliseconds - Node.js makes no guarantees about the exact timing of when
+the callback will fire, nor of the ordering things will fire in. The callback will
+be called as close as possible to the time specified.
+
 ### clearTimeout(timeoutId)
 
 Prevents a timeout from triggering.