fdc3.joinChannel docs should explain expected behavior on joinChannel

[kriswest]

Area of Issue

[ ] App Directory
[ ] API
[ ] Context Data
[ ] Intents
[ ] Use Cases
[X] Other (documentation)

Issue Description:

The documentation for fdc3.joinChannel() does not clarify that the desktop agent should immediately call any context listeners already registered via fdc3.addContextListener with the current context as defined in the spec:

When an app joins a channel it will automatically recieve the current context for that channel.

This presumably implies the same behaviour as the Channel API's getCurrentContext function, where, if a type is passed the last context of that type broadcast on the channel is returned, rather than just the last context. However, this is not clarified in either the spec or documentation.

Both the specification and documentation should be updated to make the expected behaviour clear.

[pgn-vole]

There is also a lack of precision around ad-hoc channel listening. I suppose this scenario should follow the behavior of fdc3.addContextListener.

const redChannel = await fdc3.getOrCreateChannel('red')
redChannel.addContextListener(() => {
  // Am I getting called with the last broadcasted context straight away?
})

[kriswest]

@pgn-vole We should definitely clarify the docs on that case too. I think the answer is no, your context listener will not get called right away. Rather you have to use the channel.getCurrentContext() function (https://fdc3.finos.org/docs/api/ref/Channel#getcurrentcontext) to manually retrieve it.

I believe that these differences in behavior are intentional (now that I know about them)