PIG-3670: Fix assert in Pig script

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1566518 13f79535-47bb-0310-9956-ffa450edef68
PIG-3483: Document ASSERT keyword

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1566521 13f79535-47bb-0310-9956-ffa450edef68