Display Pull Request "local checkout" Help

As outlined in https://answers.atlassian.com/questions/179848/local-checkout-of-a-pull-request-in-stash it would be a good idea to provide a helper dialog/note/message to aid users in how to locally pull down a pull request and inspect it on a machine before merging into branch.

While the suggestion of properly pulling down the source branch is the "correct" way to do it, in practice, especially in large organizations of 200-300 people split in multiple teams, it is really a hassle to constantly re-adjust permissions to forked repos, when submitting pull requests, as any given developer has multiple pull requests open at any given time, and that would introduce significant maintenance overhead for permissions.