Explain how to use custom matchers (#13957)

The test only compile because `jasmine-tests.ts` doesn't import or export
anything, and so the typescript compiler thinks it is a global definition file.
I've added a note explaining how normal users would declare their custom
matchers.

jasmine/jasmine-tests.ts

@@ -872,6 +872,14 @@ var customMatchers: jasmine.CustomMatcherFactories = {
     }
 };
 // add the custom matchers to interface jasmine.Matchers via TypeScript declaration merging
+// if your test files import or export anything, you'll want to use:
+// declare global {
+//     namespace jasmine {
+//         interface Matchers {
+//             ...
+//         }
+//     }
+// }
 declare namespace jasmine {
     interface Matchers {
         toBeGoofy(expected?: any): boolean;