Clarify differences between App and User Channel behaviour (in docs for FDC3 versions 1.1, 1.2, 2.0 and next)

[kriswest]

Area of Issue

[x] API

Issue Description:

There is a key difference in behaviour between App Channels and System/User channels in FDC3 versions 1.1, 1.2, 2.0 in that, when you join a user channel (or add a context listener when already joined to the channel) your handler immediately receives the current context of the channel (filtered by the type of the listener). However, this is not the case when working with a channel through the Channel interface. In that case, you do NOT receive the current context but can retrieve it manually with channel.getCurrentContext(/* optional context type name */).

This is currently (i.e. in 2.0) only stated in the overview:
2.0:

When an app joins a User channel, or adds a context listener when already joined to a channel, it will automatically receive the current context for that channel.

https://fdc3.finos.org/docs/api/spec#joining-user-channels

1.2

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

https://fdc3.finos.org/docs/1.2/api/spec#joining-channels

and not in the function references for either the main Desktop Agent API or Channel interface. Hence, it is often missed by both Desktop Agent developers and application developers. Additional copy should be added to past and current documentation to clarify the behavior that should be expected.

[kriswest]

Note that conformance testing additions related to this are proposed under #864 and #865