docs(migration): note that services can now return functions

Request Type: docs

How to reproduce: 

Component(s): misc core

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

noting breaking change in 1.2 where services need to correctly return the object or function it desires. 

The change mostly effects preprocessed javascript, though javascript nonetheless.

References https://github.com/angular/angular.js/commit/c22adbf160f32c1839fbb35382b7a8c6bcec2927

**Other Comments:**