Rollup merge of #137641 - kpreid:dealloc, r=Amanieu

More precisely document `Global::deallocate()`'s safety.

There is a subtlety which "other conditions must be upheld by the caller" does not capture: `GlobalAlloc`/`alloc::dealloc()` require that the provided layout will be *equal*, not just that it "fits", the layout used to allocate. This is always true here due to how `allocate()`, `grow()`, and `shrink()` are implemented (they never return a larger allocation than requested), but that is a non-local property of the implementation, so it should be documented explicitly.

r? libs

`@rustbot` label A-allocators

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