doc: improve process event headers

The headers should be handled as all others as well and just indicate
all arguments.

PR-URL: https://github.com/nodejs/node/pull/20312


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>