Improve spec and documentation of fdc3.addContentListener to mention joining channels

[kriswest]

Minor Issue

The description of the addContextListener functions from the Desktop Agent and Channel APIs is poor in both the specification and documentation and needs to be improved.

Area of Issue

[ ] App Directory
[x] API
[ ] Context Data
[ ] Intents
[ ] Use Cases
[ ] Other

Issue Description:

• Update Context Channels section of the specification to better describe the intended use of system and app channels
  • Actually define usage of broadcast and addContextListner functions
• Joining channels -> Examples section gives incorrect advice on checking current state of a system channel - it will be received automatically on joining that channel, move getCurrentContext example to Direct Listening section.
• Direct Listening and Broadcast on Channels section incorrectly refers to Channel.addBroadcastListener instead of Channel.addContextListener and should include the getCurrentContext example
• Documentation for fdc3.addContextListener should mention being joined to a channel AND receiving current context automatically when joining/adding the listener
• Documentation for Channel.addContextListener should highlight differences from fdc3.addContextListener, including the fact it doesn't auto-receive context (as there is no join fn)