Document session usage first
Document session usage first

Since we'd prefer developers to use the session method when
constructing the Client instance, document it first.

Change-Id: I8998a9962fd541bafae32b3443d7d4767da74257

sdpp/python-keystoneclient,Mercador/python-keystoneclient,magic0704/python-keystoneclient,sdpp/python-keystoneclient,klmitch/python-keystoneclient,ging/python-keystoneclient,darren-wang/ksc,darren-wang/ksc,magic0704/python-keystoneclient,klmitch/python-keystoneclient,Mercador/python-keystoneclient,ging/python-keystoneclient