Rollup merge of #137714 - DiuDiu777:doc-fix, r=tgross35

Update safety documentation for `CString::from_ptr` and `str::from_boxed_utf8_unchecked`

## PR Description​
This PR addresses missing safety documentation for two APIs:

​**1. alloc::ffi::CStr::from_raw**​

- ​`Alias`: The pointer ​must not be aliased​ (accessed via other pointers) during the reconstructed CString's lifetime.
- `Owning`: Calling this function twice on the same pointer and creating two objects with overlapping lifetimes, introduces two alive owners of the same memory. This may result in a double-free.
- `Dangling`: The prior documentation required the pointer to originate from CString::into_raw, but this constraint is incomplete. A validly sourced pointer can also cause undefined behavior (UB) if it becomes dangling. A simple Poc for this situation:
```
use std::ffi::CString;
use std::os::raw::c_char;

fn create_dangling() -> *mut c_char {
    let local_ptr: *mut c_char = {
        let valid_data = CString::new("valid").unwrap();
        valid_data.into_raw()
    };

    unsafe {
        let _x = CString::from_raw(local_ptr);
    }
    local_ptr
}

fn main() {
    let dangling = create_dangling();
    unsafe {let _y = CString::from_raw(dangling);} // Cause UB!
}
```

​**2. alloc::str::from_boxed_utf8_unchecked**​

- `ValidStr`: Bytes must contain a ​valid UTF-8 sequence.

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