doc: improve common.mustCall() explanation

PR-URL: https://github.com/nodejs/node/pull/10390


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>