Try to clarify GlobalAlloc::realloc documentation comment.
This PR tries to improve the documentation of [GlobalAlloc::realloc](https://doc.rust-lang.org/alloc/alloc/trait.GlobalAlloc.html#method.realloc) with two aspects:
1. Explicitly mention that `realloc` preserves the contents of the original memory block.
2. Explicitly mention which layout should be used to deallocate the reallocated block.
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