Note that timer_reset() must not be executed before timer expired

I've seen several people (including me) having problems with the behavior of `timer_reset()` when it is invoked before the timer expired.

The problem was already discussed in #293 more detailed.

In my opinion adding a simple note should be the easiest solution so far as it does not introduce further computation overhead would but gives a hint for (doc-reading) users.


I think that this documentation could be further improved by writing "This function should not be called before the timer expired."