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