Add documentation for `machine.Counter` and `machine.Encoder` as currently
implemented by the esp32 port, but intended to be implemented by other
ports.
Originally authored by: Ihor Nehrutsa <Ihor.Nehrutsa@gmail.com> and
Jonathan Hogg <me@jonathanhogg.com>.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
