doc: improve common.mustCall() explanation

Trott · MylesBorins · commit 32877d96c27a · 2017-01-24T00:00:04.000-08:00
PR-URL: #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>
diff --git a/test/README.md b/test/README.md
@@ -290,8 +290,11 @@ Array of IPV6 hosts.
 ### mustCall(fn[, expected])
 * fn [&lt;Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
 * expected [&lt;Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type) default = 1
+* return [&lt;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` [&lt;Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type)
