Add clarifying comment to Lens test mock.

x-pack/legacy/plugins/lens/public/indexpattern_plugin/operations/definitions/date_histogram.test.tsx

@@ -15,6 +15,15 @@ import { createMockedIndexPattern } from '../../mocks';
 import { IndexPatternPrivateState } from '../../types';
 
 jest.mock('ui/new_platform', () => {
+  // Due to the way we are handling shims in the NP migration, we need
+  // to mock core here so that upstream services don't cause these
+  // tests to fail. Ordinarly `jest.mock('ui/new_platform')` would be
+  // sufficient, however we need to mock one of the `uiSettings` return
+  // values for this suite, so we must manually assemble the mock.
+  // Because babel hoists `jest` we must use an inline `require`
+  // to ensure the core mocks are available (`jest.doMock` doesn't
+  // work in this case). This mock should be able to be replaced
+  // altogether once Lens has migrated to the new platform.
   const { coreMock } = require('src/core/public/mocks'); // eslint-disable-line @typescript-eslint/no-var-requires
   return {
     npSetup: {