Add docstring for exception handlers of session

In code review, some projects are using try/except within the
context managed by session. That's wrong, add the note in
docstring, hope others put try/except in right place.

Database Errors like IntegrityError will be raised in session's
__exit__ handler, and any try/except within the context managed
by session will not be triggered. And catching other non-database
errors in the session will not trigger the ROLLBACK, so exception
handlers should always be outside the session, except developer
want to commit parts of stuff on purpose.

Closes-Bug: #1262065

Change-Id: I23223fbe9b6d9fa8e92e59894fb3b1fa195be159
Merge "Add docstring for exception handlers of session"