Fix missing safety in {ptr,NonNull}::as_mut

[DiuDiu777]

In this PR:

1. The initialization precondition for ptr::{ as_mut, as_ref, as_mut_unchecked, as_ref_unchecked } and NonNull::{ as_mut, as_ref } have been moved under the safety contracts.
2. Specified the potential aliasing risks of APIs that return a reference, including ptr::{ as_mut, as_ref, as_mut_unchecked, as_ref_unchecked, as_uninit_mut, as_uninit_ref } and NonNull::{ as_mut, as_ref, as_uninit_mut, as_uninit_ref }. The following code passes the compiler's exclusivity checks, but Miri reports undefined behavior.

fn main() {
    use std::ptr::NonNull;
    let mut x = 0u32;
    let mut ptr = NonNull::new(&mut x).expect("null pointer");
    let x_ref = unsafe { ptr.as_mut() };
    unsafe { 
        *ptr.as_ptr() += 2;  
    }
    *x_ref += 2;
    println!("Final value: {}", x_ref);
}

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[Noratrieb]

what's the motivation for the two changes? I think having a note about aliasing and uninitialized memory is useful, but I'm not sure if they should be part of the # Safety section like that.

[DiuDiu777]

Thanks for reviewing!

• For Init, it's an exact precondition of these APIs. We’re moving this note into # Safety because that’s where all preconditions should live – it helps devs see the rules in one place and aligns with how other unsafe APIs document their contracts. Plus, it’ll make community to build safety contracts more fully in the future (like supporting contract verification project).
• For alias, these APIs quietly require exclusive access (&mut guarantees) to avoid UB, but it wasn’t written down. For example, pointer::as_uninit_slice explicitly says this, but these APIs didn’t. We spotted real UB in code that violates this (see above code), so we’re fixing the docs to match what the APIs actually require.