The Changed() method did not actually return anything, and in fact the
no_reply flag for that message was set.
Similarly, the Release method does not expect a reply.
When the SignalLevelAgent doc blurb was moved to station-api.txt it
seems the interface was changed to Station.SignalLevelAgent in some
places but not in most and not in the code. Also fix the pointers to
the doc file.
Station was really a misnomer, it should have been AccessPoint. Also
mark the API as experimental. It will be moved to the Diagnostics
interface eventually.
