Add an example of using v3 client with sessions

Added a bit more information to the 'using-api-v3' section, about
using sessions instead of passing in arguments.
This overlaps with what is documented in 'using-sessions' but I
think it's still handy as most folks will likely go here first.

Change-Id: I352bda49e01362d3a5da9a79ceb34748552ef9ec
Merge "Add an example of using v3 client with sessions"