doc: discourage error event
Refs: https://github.com/nodejs/node/pull/37237#issuecomment-774119972
Refs: https://github.com/nodejs/node/pull/37237
Basically - the "error" event is being removed from the code - it was never documented on process
but in order to be cautious the change is still semver-major.
This PR changes the wording to indicate the future behaviour.
Bikeshedding/suggestions welcome