doc/reference/drivers: document device-specific API extensions

This PR updates the documentation to cover a technical solution to
providing a driver instance that extends the functionality of a
subsystem API.  The solution described was mooted in Zephyr PR #17072
and approved by the Technical Steering Committee during its 2019-08-07
meeting.

Relates-to: #11993
Signed-off-by: Peter A. Bigot <pab@pabigot.com>