docs(error/$compile): add note that html comments can cause Invalid Template Root error

Request Type: docs

How to reproduce: 

Component(s): $compile

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

Users with comments in their templates can overlook the fact that these may be the cause of an Invalid Template Root error.  This note helps them identify when html comments are the cause of this error.

**Other Comments:**