clarify requirements of byte_{offset,add,sub} for zero-sized referents

[jswrenn]

The safety documentation on core::ptr presents this rule:

For operations of size zero, every pointer is valid, including the null pointer.

However, due to the implementation details of byte_{offset,add,sub}, which involve intermediate operations on non-zero-sized referents, this rule does not apply to these methods.

This commit clarifies extends the over-arching rule with an "unless otherwise noted" caveat, and clarifies the documentation of byte_{offset,add,sub} to note that the only valid count for zero-sized referents is presently 0 (though I wonder if this requirement might be relaxed).

cc @rust-lang/opsem

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[saethlin]

though I wonder if this requirement might be relaxed

I don't think it should be, I think of byte_ operations as doing arithmetic on the address while ignoring the pointee type. The whole point is ignoring the pointee type.

So I'm not sure what other behavior you would want for these methods on ZST pointees. Making them do nothing would be a breaking change.