You can try this out in the API Explorer to see what the parameters would look like. With Python, the API call might look something like:
service.userUsageReport().get(userKey='all',
date='2013-12-02',
parameters='docs:num_docs_shared_outside_domain').execute()
if you remove the parameters field entirely, you'll get all of the results from the user usage report, including the parameters simply makes things more efficient (and presumably faster) by limiting the results to that single parameter.