Rename the client API docs

Since developers want to use the APIs, the docs should be more
enticing and say that it describes how to use the APIs.

Also, called it "V3 Client API" since this reads better than
"Client V3 API" and it matches the order in the module path.

Change-Id: I79dd6f6891bf48b477b35157256a0219426d171c
Merge "Rename the client API docs"