Add an explicit example of constraining "transaction.search" to the webhook documentation
Summary:
Depends on D19509. Ref T13151. See PHI725. `transaction.search` supports "constraints" and the documentation mentions it in passing, but doesn't really show how to do it, and the method is not automatically self-documenting because it isn't a "real" `*.search` method.
Add an example of how to use the `contraints` parameter to retrieve information about only the relevant transactions.
Also remove a refernece to `requestb.in`, which is now defunct.
Test Plan: Called `transaction.search` with similar parameters.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13151
Differential Revision: https://secure.phabricator.com/D19510