docs: document callback_unwrap attribute

[TobieTom]

Fixes #1303

Adds documentation for the callback_unwrap attribute under the #[near] macro docs, including:

• Description of functionality and usage
• Integration with PromiseOrValue
• Cross-contract factorial example
• Rust doc annotations and references

---

For more details, open the Copilot Workspace session.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.33%. Comparing base (9596835) to head (8f7cf09).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1313      +/-   ##
==========================================
+ Coverage   80.22%   80.33%   +0.10%     
==========================================
  Files         104      104              
  Lines       14844    14829      -15     
==========================================
+ Hits        11909    11913       +4     
+ Misses       2935     2916      -19     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[dj8yfo]

overall this is the right direction, but all examples should at least compile (or run).

the second one isn't very clear what it's about, and it doesn't compile, which is a sign
that it's incorrect and/or incomplete.

this can be iterated quicker locally with cargo test --doc near-sdk/src/lib.rs

[dj8yfo]

for task #1312 (NOT in scope of current pr) it can be mentioned that the attribute works by doing the following:

#[no_mangle]
pub extern "C" fn factorial_mult() {
 ....
    let data: ::std::vec::Vec<u8> = match ::near_sdk::env::promise_result(0u64) {
        ::near_sdk::PromiseResult::Successful(x) => x,
        _ => ::near_sdk::env::panic_str("Callback computation 0 was not successful"),
    };
    let cur: u32 = match ::near_sdk::serde_json::from_slice(&data) {
        Ok(deserialized) => deserialized,
        Err(_) => ::near_sdk::env::panic_str("Failed to deserialize callback using JSON"),
    };
    let contract: CrossContract = ::near_sdk::env::state_read().unwrap_or_default();
    let result = CrossContract::factorial_mult(&contract, n, cur);
    
    ...

with links to corresponding [near_sdk::env] host-functions used

[dj8yfo]

it was also missed that there should've been a link from near_annotations module to this newly created anchor