Update promise documentation

[kfarnung]

Needs to wait for chakra-core/ChakraCore#5131 to land.
Needs to wait for chakra-core/ChakraCore#5138 to land.

[MSLaguana]

Looks like you've just copy/pasted from the other method; this should be a JsValueRef out parameter I believe.

[liminzhu]

Thanks for keeping the docs updated!

Comments on documentation

1. Consider documenting behavior of JsGetPromiseResult when promise is pending (JsErrorInvalidArgument currently) and rejected (error object). At least the pending behavior isn't very obvious.

I hope it's not too late for comments on those APIs themselves

1. Most of our enums don't use _. Consider something like JsPromiseStatePending.
2. This is debatable - I'm not sure about JsGetPromiseResult returning JsErrorInvalidArgument from pending promises. Intuitively, I only expect this if you throw in something other than promise. You can have a valid argument (a promise) but just catch it in a bad state. I'd suggest returning JS_INVALID_REFERENCE if you consider this a valid and successful operation or a new error code like JsErrorPendingPromise for a failed op.

[kfarnung]

@liminzhu I made the documentation changes to resolve your feedback, but it's pending another change to ChakraCore before this can land.

[kfarnung]

Opened chakra-core/ChakraCore#5138

[kfarnung]

@liminzhu @MSLaguana Any other concerns with this documentation?

[liminzhu]

LGTM, thx!