Fix Box's doc for aliasing rules

[DiuDiu777]

We noticed there are four spots in Box's documentation where the safety notes are incomplete. According to Rust's stacked borrows rules, when APIs like from_raw take ownership of a pointer's memory, that original pointer shouldn't be touched anymore. Other similar APIs make this clear, like Vec::from_raw_parts specifically says:

The ownership of ptr is effectively transferred to the Vec<T> which may then deallocate, 
reallocate or change the contents of memory pointed to by the pointer at will. 
Ensure that nothing else uses the pointer after calling this function.

But Box's docs are currently missing this important warning.

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r? @jhpratt

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[jhpratt]

I have no idea if this is correct…Ralf is probably the person that knows this best.

r? @RalfJung

[RalfJung]

Please don't start a sentence with "and", that's not proper writing. Also, "enforce the aliasing rule" is unclear, at least it should say "enforce the aliasing rules", but even then it's still not clear what those rules are. Maybe just copy the wording from Vec::from_raw_parts?

Also, it's not true that the raw pointer may never be used again. If you mem::forget the Box, you can use the raw pointer again. But the same is true for Vec so maybe we deliberately don't talk about this in the docs? Cc @rust-lang/opsem