document safety requirements for atomic intrinsics

[yilin0518]

This PR serves as a complement to PR #162854 by adding safety docs for the remaining atomic operations. These operations pass raw pointers as arguments and are marked unsafe, so I think it is necessary to explain why they are unsafe.

For atomic_cxchg, atomic_cxchgweak, atomic_xchg, these APIs have the same process whether T is an integer or a pointer type.

For atomic_xadd, atomic_xsub, atomic_and, atomic_or, atomic_xor, these APIs correspond to different operations in core/src/sync/atomic.rs for T is an integer or a pointer type.

For atomic_max, atomic_min, atomic_umax, atomic_umin, they only process when T is an integer (signed or unsigned) type.

For atomic_nand, there doesn't exist a method in AtomicPtr<T>, so I don't link the related method.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

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