test,doc: document `crashOnUnhandledRejection()`

Add documentation for `common.crashOnUnhandledRejection()`.

Ref: a9c2078a60 (r113737423)
PR-URL: https://github.com/nodejs/node/pull/12699
Backport-PR-URL: https://github.com/nodejs/node/pull/13103
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>

test/README.md

@@ -174,6 +174,13 @@ A stream to push an array into a REPL
 
 Blocks for `time` amount of time.
 
+### crashOnUnhandledRejection()
+
+Installs a `process.on('unhandledRejection')` handler that crashes the process
+after a tick. This is useful for tests that use Promises and need to make sure
+no unexpected rejections occur, because currently they result in silent
+failures.
+
 ### ddCommand(filename, kilobytes)
 * return [<Object>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)