Timer/Event block documentation

[DavidWhaleMEF]

We've had some queries from users wishing to use the micro:bit to control 4 PWM channels, e.g. for quadcopter control.

The nRF51 datasheet on table 12 states there are 3 TE units, each with 4 channels.

It would be helpful to have some additional tech information in our tech docs stating how each of the timer peripherals are allocated on the micro:bit (in fact, as a wider thing, how about a detailed breakdown of how every nRF51 on board peripheral is mapped to features?) so that it would be easier for people to answer these sorts of questions themselves.

In particular, with full knowledge of how timer TE units are allocated for each Timer, it would be much easier to assess whether going in at the mbed level (or throwing mbed away completely and coding direct to the core) would support various project development ideas.