Some beginners are confused about the "extern" section, so I added an
explanation comment to all examples. Furthermore, using the UARTx
interrupts when UART is actually being used in the same example may be
confusing, so I changed them all to SSI0/QEI0.
The example above this in the documentation states
```
// semantically, the monotonic timer is frozen at time "zero" during `init`
// NOTE do *not* call `Instant::now` in this context; it will return a nonsense value
let now = cx.start; // the start time of the system
```
It results in weird scheduling issues, but still eventually works. `cx.start` is much more reliable.
Relates to https://github.com/rtfm-rs/cortex-m-rtfm/issues/196
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
