Updates the main description of the `api/set-typing-status` endpoint
for the new fields in the register response for the typing start,
stop, expired time intervals. Previously these were hardcoded by
the client side code and not the server side code.
Also updates the developer documentation for typing indicators in
the subsystems docs. This refreshes a few parts of that doc that
were already out of date, as well as adds the information about
the new register response fields noted above.
29bb346480