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>

test/README.md

@@ -290,8 +290,11 @@ Array of IPV6 hosts.
 ### mustCall(fn[, expected])
 * fn [<Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
 * expected [<Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type) default = 1
+* return [<Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
 
-Number of times `fn` should be called.
+Returns a function that calls `fn`. If the returned function has not been called
+exactly `expected` number of times when the test is complete, then the test will
+fail.
 
 ### nodeProcessAborted(exitCode, signal)
 * `exitCode` [<Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type)