docs(ngMock): fixes in flush() documentation

The docs for the `flush()` method contained a few grammatical
errors and were awkwardly worded. Change the explanation of
the method to remove errors and read more naturally.

Closes #4886

src/ngMock/angular-mocks.js

@@ -960,13 +960,12 @@ angular.mock.dump = function(object) {
  *
  * # Flushing HTTP requests
  *
- * The $httpBackend used in production always responds to requests with responses asynchronously.
- * If we preserved this behavior in unit testing we'd have to create async unit tests, which are
- * hard to write, understand, and maintain. However, the testing mock can't respond
- * synchronously because that would change the execution of the code under test. For this reason the
- * mock $httpBackend has a `flush()` method, which allows the test to explicitly flush pending
- * requests and thus preserve the async api of the backend while allowing the test to execute
- * synchronously.
+ * The $httpBackend used in production always responds to requests asynchronously. If we preserved
+ * this behavior in unit testing, we'd have to create async unit tests, which are hard to write,
+ * to follow and to maintain. But neither can the testing mock respond synchronously; that would
+ * change the execution of the code under test. For this reason, the mock $httpBackend has a
+ * `flush()` method, which allows the test to explicitly flush pending requests. This preserves
+ * the async api of the backend, while allowing the test to execute synchronously.
  *
  *
  * # Unit testing with mock $httpBackend