doc: add details for #[ext_contract] macro at the site of re-export in near-sdk

[dj8yfo]

a skeleton of this was added in #1295, but it needs more info about which methods are provided
and how to view the signatures of stuff obtained via this macro with cargo doc on contract crate (if possible).

And what's the purpose of each added method.

Similar to #1308

[Haroldwonder]

Let me try this one! I am a developer proficient at using Rust to build robust and efficient products. I am a new contributor and will like to try this one.

[dj8yfo]

@Haroldwonder yeap, sure, go ahead