Updated non-blocking display documenation and added Changelog entry

[videbar]

Fixes #109

[winksaville]

@videbar as a retired programmer but noob to microbit, I don't feel fully qualified to give a formal review, but this LGTM and can approve this if you'd like.

[therealprof]

Hm, need to check why the CI passes are failing. Seems the doc tests are invalid.

[videbar]

Hm, need to check why the CI passes are failing. Seems the doc tests are invalid.

As far as I can tell, the error seems to be in this portion of blocking.rs:

//! The coordiante system is oriented so the 'bottom' (x,4) row is the edge with the edge
//! connector. That means that
//!
//! ```no_run
//! display.show(
//!    &mut timer,
//!    [
//!        [0, 0, 1, 0, 0],
//!        [0, 1, 1, 1, 0],
//!        [1, 0, 1, 0, 1],
//!        [0, 0, 1, 0, 0],
//!        [0, 0, 1, 0, 0],
//!    ],
//!    1000,
//!);
//! ```

I'm not super familiar with doc tests, but to me it seems like the CI is trying to compile the example above but fails because it can't find display or timer.

[videbar]

Upon further inspection, it seems like the faulty doc test was recently introduced in #119. For some reason, it seems like the CI only checked the changelog in that PR and the error wasn't caught. I think it should be enough to add

//! # use microbit_common as microbit;
//! # use microbit::{
//! #     Board,
//! #     hal,
//! #     display::blocking::Display,
//! # };
//! let board = Board::take().unwrap();
//! let mut timer = hal::Timer::new(board.TIMER0);
//! let mut display = Display::new(board.display_pins);

at the beginning of the example.