Explaining empty payloads in each binding

[egekorkan]

Coming partly from w3c/wot-scripting-api#430 and the Scripting API call on 26.09.22:

Currently, the TD spec describes empty payloads of a protocol by not describing it. For example, for a POST request with no payload or an event that does not need any payload (like buttonPressed). However, this empty payload can be done in different ways in different protocols. In HTTP, it is empty body, in MQTT it is empty payload in publish but a subscribe MUST contain a payload apparently (http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718066, just learned it). I am guessing that it can be an empty payload in the end but it still needs encoding.

In any case, I think that we should add explaining this empty payload behavior for all bindings.