Add role assignments as concept in Client API V3 docs
Add role assignments as concept in Client API V3 docs

Change-Id: Ia04e51d98b5f00a28d94c24ed7665fcead3f6507
Closes-Bug: #1325739

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