Add role assignments as concept in Client API V3 docs

Change-Id: Ia04e51d98b5f00a28d94c24ed7665fcead3f6507
Closes-Bug: #1325739
Changes exception raised by v3.trusts.update()

The client should raise a Client error, instead of
a Server one.

The MethodNotImplemented exeception handles calls
made to not supported methods.

Change-Id: I0e2a8c5e1e53b0f17b886aa693c922bc193bb882
Closes-Bug: #1325755