How do I use the joins feature of the new API
The JIRA Issue for this feature https://issues.civicrm.org/jira/browse/CRM-17795 reads


  This adds support for joins in api.get operations to fetch information
  from more than one table.
  
  It also adds ACL enforcement to the Contact and related (email,
  address, phone, etc) apis for get, create &amp; delete actions.


The JIRA issue links to a couple of pull requests; pull request 7565 includes a comment that has screen shots and descriptions (and one join example).