Clarify expected behaviour of IntentResolution.getResult() when IntentHandlers resolve to void

[kriswest]

Area of Issue

[x] API

Issue Description:

The documentation of the IntentHandler type introduced in 2.0 and related behaviour from the IntentResolution.getResult() API call, when a void result is returned, is open to interpretation due to a confusing description. Clarify the description to ensure its easy to understand the expected behavior.

Additional Context:

One of the use-cases for getResult() discussed when it was introduced was knowing when the intent handler has completed. This should be possible even when the intent handler is returning void and shouldn't appear to be an error. However, the description of IntentHandler and the ResultError.NoResultReturned don't quite match up (You might resolve getResult() to void OR reject with ResultError.NoResultReturned. It was not intended that the promise rejects with an error on expected behavior.

As this was not clear in 2.0 it was suggested that the 2.0 conformance tests allow for either output, while 2.1 fixes the description and standardizes on resolving to void.