Fix mixed up worker signal handling

Commit 81241907ffcf94517ffa14b8427205906b61b540 changed the signal
handling by switching the roles of `TERM` and `QUIT` for the arbiter
so that `TERM` is graceful and `QUIT` is not.

At the time, workers performed graceful shutdown on `QUIT` and quick
shutdown on `TERM` and `INT`. This behavior was also changed so that
`QUIT` (and `INT`) cause a quick shutdown and `TERM` is graceful.

However, the documentation incorrectly reversed the roles of the worker
signals and the arbiter was not updated to use the correct signals.

This commit fixes the documentation and the arbiter signals.
Merge pull request #745 from tilgovi/worker-signals-mixup

Fix mixed up worker signal handling