Update documentation of as_ptr function of Atomic$Int to clarify circumstances of usage

[madhav-madhusoodanan]

Context

The documentation of as_ptr mentions that "operations on it must be atomic". This may confuse developers, since non-atomic reads may be safely performed in certain circumstances (eg: when they do not lead to a data race).

The core message is that non-atomic accesses are UB if they cause a data race.

This update ensures such clarity on the circumstances of usage of the as_ptr function.

Associated Issue

• Atomic*::as_ptr unnecessarily restricts operations on the pointer to atomic operations #138246

cc: @RalfJung @briansmith

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[briansmith]

Thanks for taking this on.

[madhav-madhusoodanan]

I've added an example where there is no unsafe block being used.
Do let me know if its a good example.

[RalfJung]

@rustbot author

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Reminder, once the PR becomes ready for a review, use @rustbot ready.

[madhav-madhusoodanan]

@rustbot ready