Document `onServer`?

I'm using onServer to be able to hook into express before the batch route is added, so I can do something like this: https://stackoverflow.com/a/21858212/4396258

(Context: we want to just restart workers if they consume too much memory, but specifically at the end of a request so we don't drop drop any requests)

It's either this or adding a workerShouldExit method as a config? What do you think? Can I safely use the onServer method, or is this subject to removal?

(Also on the subject of undocumented arguments - what about documenting context too? It's useful for collecting stats.)

@ljharb @goatslacker