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:

Subtasks for each of the following should be created and handled individually:

• callback_unwrap
  • should mention it can be combined with callback_result after callback_result is documented
  • should mention #[callback_vec] argument includes other args with #[callback_unwrap] #1320 after callback_vec is documented
• callback_result
  • should add contract method+asserts to https://github.com/near/near-sdk-rs/blob/master/examples/callback-results/src/lib.rs#L50-L54 (done in 7fe8625 in doc: impl details of #[callback_unwrap] #1319 )
  • should mention it can be combined with callback_unwrap
  • should mention #[callback_vec] argument includes other args with #[callback_unwrap] #1320 after callback_vec is documented
• callback_vec
  • should mention #[callback_vec] argument includes other args with #[callback_unwrap] #1320

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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

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Bugs discovered:

• #[callback_vec] argument includes other args with #[callback_unwrap] #1320

[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?