Added minor clarification to specification of GlobalAlloc::realloc.
The specification of `realloc` is slightly unclear:
```
/// * `layout` must be the same layout that was used
/// to allocate that block of memory,
```
https://github.com/rust-lang/rust/blob/master/src/libcore/alloc.rs#L541-L542
In the case of an `alloc` or `alloc_zeroed` this is fairly evidently the `layout` parameter passed into the original call. In the case of a `realloc`, this I assume is `layout` modified to contain `new_size`. However, I could not find this case specified in the documentation. Thus technically in a sequence of calls to `realloc`, it would be valid to provide the second call to `realloc` the same `layout` as the first call to `realloc`, which is almost certainly not going to be handled correctly.
This PR attempts to clarify the specification.