Document N1QL-JS UDF upgrade behaviour from 7.0.x to 7.1.x

As noted in MB-50350 , the following behaviour needs to be documented:

Any attempt to execute function with N1QL-JS UDFs replicated from a 7.1.x node will fail with "no library <name> found in worker" on a 7.0.x node.

However, JS-UDFs without N1QL statements will run on both - 7.1.x and 7.0.x nodes.