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.