Doc fix

[DiuDiu777]

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.

[rustbot]

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[Sky9x]

not the pointer, but the memory it points too

[Sky9x]

This is not a safety requirement. Box<[u8]> is really Box<[u8], Global>, so passing a box with a custom allocator to this function is prevented by type check.

[DiuDiu777]

Yes, you are right. I will fix these.