=con #3228 Adjust structure of TimerBasedThrottler ScalaDoc

* The documentation was good, but some parts were "hidden" by separating
  it two places. I understand the original reason for the separation but
  it might be easier for the user (as reported in the ticket) to have
  everything in one place.
Merge pull request #1997 from akka/wip-3228-doc-TimerBasedThrottler-patriknw

=con #3228 Adjust structure of TimerBasedThrottler ScalaDoc