BigQuery: document proper use of run_async_query/QueryJob/QueryResult

If I run an async query using run_async_query(), get a QueryJob from it and a QueryResult from that by calling job.query_results(), the rows attribute of QR is empty. It seems that when the GET request is sent a parameter maxResults=0 is automatically added, in the client code, which explains why rows is empty.

But this is confusing and not how it behaves for sync queries. I think it should allow getting some rows in that first GET request.

Can you please document the proper use of async queries and QueryResult? Thank you.