Additional variants of `interleave` `union` optarg

https://github.com/rethinkdb/docs/issues/1021#issue-133851169 didn't mention all possible types for the interleave option.

Apart from true, false and field_name, it can also be:

• A lambda function which is applied to each element in the input streams. The interleaving is determined by comparison on the value returned by the lambda function.
• An array of field names
• An array of lambda functions
• r.desc(field name) or r.desc(lambda x: ...) or r.desc([field1, field2, ...]) or r.desc([lambda x: ..., lambda x: ..., ...])

For the last one I'm a little bit fuzzy on whether the r.desc must be around the array, or if it can be applied to particular elements inside the array. @nighelles can you clarify please?