Compass and accelerometer docs need updating to dal v2.1 instantiation mechanism

[jaustin]

With the micro:bit motion sensor update (https://support.microbit.org/support/solutions/articles/19000087020-micro-bit-motion-sensor-hardware-change ) we have changed the way the motion sensors need to be initialised and this needs to go into the docs here.

https://support.microbit.org/support/solutions/articles/19000084312-micro:bit-motion-sensor-hardware-changes-for-editor-developers contains some more details but now that the process is complete we should document things clearly

[microbit-carlos]

I think this issue can be closed, is that right @microbit-sam?

[microbit-sam]

Yep, I've updated these docs