refactor(ngMock window.inject test): add testInjectCaller() usage comment

The results of this function, when called outside of a specific test spec
context, should not be reused in multiple tests as they may have stored
state that can cause unwanted test spec interaction. This explains why we
may need to wrap some tests into their own separate test suites instead
of grouping them all under a single shared one.

Author: jurko-gospodnetic

test/ngMock/angular-mocksSpec.js

@@ -929,6 +929,9 @@ describe('ngMock', function() {
         return !!error.stack;
       })();
 
+      // function returned by inject(), when called outside of test spec
+      // context, may have stored state so do not reuse the result from this
+      // call in multiple test specs
       function testInjectCaller() {
         var shouldThrow;
         var injectingCall = (function internalInjectCaller() {