Add missing parameters to Client's docstring

Client's constructor receives keyword arguments (**kwargs), which are
then passed to an HTTPClient. Since HTTPClient receives numerous
parameters, Client's docstring describes which parameters are expected
and their meanings. This patch adds 2 parameters which HTTPClient
receives but weren't previously documented in Client's docstring.

Change-Id: I12e520f6e26c3aa7c76b190c3e2125d77c71a19a