Expand javadoc on execSelect(), execConstructTriples() and execDescribeTriples() to highlight the fact that these methods typically return query plans rather than actually fully execute the query.

Queries are only actually evaluated as a user iterates over the returned iterators.

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1458558 13f79535-47bb-0310-9956-ffa450edef68