Clarify guarantees for `Vec` and `String` allocation

[SimonSapin]

Similar to what #58183 did for Box, I think we should document a guarantee that Vec<T> and String are allocated through the standard library’s global allocator.

Namely:

• It is valid (under some conditions) to pass a pointer allocated with std::alloc::Global (or corresponding free functions in std::alloc) to Vec::from_raw_parts.

• It is valid (under some conditions) to deallocate or reallocate with std::alloc::Global (or corresponding free functions in std::alloc) a pointer from Vec::into_raw_parts (or from ManuallyDrop + Vec::as_mut_ptr).

Conditions include using the appropriate std::alloc::Layout, properly transferring ownership, etc.

Perhaps the documented guarantees should also extend to using NonNull<T>::dangling for zero-size allocations. (And similarly for Box.)

@rust-lang/libs, any throughts?

[alexcrichton]

I personally think that this is a useful enough guarantee while also being flexible enough for us in the standard library that this should be fine to document.