doc: document callback_... family of near-macro sub-attributes

[dj8yfo]

Relates to #1265 and #1299

E.g. callback_result is mentioned was mentioned in draft of #1299 in handle_result documentation, but there's no entry for it itself:

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it has been noticed in scope of #1313 (comment) that doc examples for these attributes should set be set up with lower-level host functions as documentation for #1308 isn't available too, so there's little point in explaining attributes with other vague unexplained concepts

[7-falseparfait]

I’d like to resolve this.

[TobieTom]

Holla. It's Tobias again. Can I take over this issue?

[dj8yfo]

@TobieTom i think it's too big. Let's try 1 attribute at a time. Will you be up to doing #1303?