PIG-3483: Document ASSERT keyword

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1529318 13f79535-47bb-0310-9956-ffa450edef68
PIG-3483: Document ASSERT keyword

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1529319 13f79535-47bb-0310-9956-ffa450edef68