doc: improve documentation for util.deprecate()

Improve documentation for `util.deprecate()`. In particular, provide
complete function signature, document arguments, and document return
value.

PR-URL: https://github.com/nodejs/node/pull/16393


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>