Clarify database session documentation.

Summary:
Don't use new_db_session() in ./inbox.

Whenever we can, we should be using session_scope()
which will properly rollback transaction failures
and close the session when it goes out of scope.

Test Plan: Ran ./inbox.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D15

Conflicts:
	tools/regen-sanitized