CPS-1526 Fix response message for PATCH operation

CPS-1526 Fix response message for PATCH operation

- Fixed the Response code and message when trying to update multiple
  data trees at once
- Response code changed to 400
- Since the patch operation across multiple data trees is not
  supported when xpath is set to root node xpath, appropriate message is
  returned in response
- Existing functionality of updating one data node with root node xpath
  remains as it is.
- Updated API documentation and release notes

Issue-ID: CPS-1526
Signed-off-by: arpitsingh <as00745003@techmahindra.com>
Change-Id: I5d25a06bb5d407316ccfb2f85877cbe56a9f6f31