Document (u)array

[microbit-matt-hillsdon]

Support: https://support.microbit.org/a/tickets/56135

The (u)array module is not documented. This is because it isn't documented in the micro:bit MicroPython documentation, so the type stubs also lack documentation which was ported from there.

See upstream MicroPython docs: https://docs.micropython.org/en/v1.14/library/uarray.html