For each type of request, what are acceptable return vals for callback and args for respond

[aoberoi]

Description

It's not clear on which types of requests, which values can be returned in the callback and/or passed to the respond() function. I've started to build a table to at least describe the current state of things. This has led to uncovering some behaviors that I think are bugs or unintended, but at least we should describe them.

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Type of Request	cb(): Message	cb(): Promise<Message>	respond(m: Message)	respond(m: Promise<Message>)	Notes
Message Action (Button, Select) incl. embeded in App Unfurl	synchronous response	if resolved under timeout, synchronous response; else response_url request	response_url request (can be used several times)	error: axios will likely throw
Dialog Submission	synchronous response	synchronous response	response_url request (can be used several times) - not a replacement, used as follow up	error: axios will likely throw	A warning is logged if the synchronous response takes longer than the timeout
Menu Options	synchronous response	synchronous response	-	-	A warning is logged if the synchronous response takes longer than the timeout

Edit: updated according to changes in #28

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