test,doc: document `crashOnUnhandledRejection()`

Add documentation for `common.crashOnUnhandledRejection()`.

Ref: https://github.com/nodejs/node/pull/12489/files/a9c2078a60bc3012dc6156df19772697a56a2517#r113737423
PR-URL: https://github.com/nodejs/node/pull/12699


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>