Merge #255

255: more monotonic timer docs r=nils-grepit a=japaric

covers

- initialization and configuration of the timer; this is now a responsibility of
  the application author
- correctness of `Monotonic::now()` in `#[init]`
- safety of `Monotonic::reset()`

closes #251
cc @jonas-schievink

(EDIT: yay, pull request number 0xFF)

Co-authored-by: Jorge Aparicio <jorge@japaric.io>

book/en/src/by-example/timer-queue.md

@@ -34,6 +34,10 @@ first appear in the `schedule` argument of the context attribute. When
 scheduling a task the (user-defined) `Instant` at which the task should be
 executed must be passed as the first argument of the `schedule` invocation.
 
+Additionally, the chosen `monotonic` timer must be configured and initialized
+during the `#[init]` phase. Note that this is *also* the case if you choose to
+use the `CYCCNT` provided by the `cortex-m-rtfm` crate.
+
 The example below schedules two tasks from `init`: `foo` and `bar`. `foo` is
 scheduled to run 8 million clock cycles in the future. Next, `bar` is scheduled
 to run 4 million clock cycles in the future. Thus `bar` runs before `foo` since