Add Backend documentation

[alecgibson]

This change adds Backend documentation to methods that provide
functionality not offered by a client.

[coveralls]

Coverage increased (+0.006%) to 95.954% when pulling 589924b on backend-docs into 0b65164 on master.

[coveralls]

Coverage remained the same at 95.948% when pulling 09b1cb3 on backend-docs into 0b65164 on master.

[alecgibson]

We should add some stub documentation for Agent, mostly to document connection.agent to get a handle on an agent, and agent.custom for middleware things. We probably don't need to document the methods.

Agents and Connections are one-to-one. Agents represent the client's state on the server-side. On the server, the Agent has a direct reference to the Connection, and to the Backend (through a direct stream). A client-side Connection does not have a reference to an Agent.