Clarify as_uninit_mut may point to uninitialized memory

[yilin0518]

Tracking issue: #75402

This PR clarifies the safety documentation for *mut T::as_uninit_mut.

Unlike as_mut, this method creates a reference to MaybeUninit<T>, so the source pointer may point to uninitialized memory. The pointer still must either be null or satisfy the pointer-to-reference conversion requirements.

After adding the supplementary information, the safety documentation of *mut T::as_uninit_mut is consistent with *mut T::as_uninit_ref, *const T::as_uninit_ref and std::ptr::NonNull::as_uninit_mut.

This is a documentation-only change.

Tests were not added because this only updates API docs.

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