PIG-3129: Document syntax to refer to previous relation (rjurney via jcoveney)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1447338 13f79535-47bb-0310-9956-ffa450edef68
PIG-3098: Add another test for the self join case (jcoveney)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1447394 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' of https://github.com/apache/pig into trunk