docs(marker/copy): clarify that `&T` is also `Copy`

In the current documentation about the `Copy` marker trait, there is a section
about "additional implementors", which list additional implementors of the `Copy` trait.
The fact that shared references are also `Copy` is mixed with another point,
which makes it hard to recognize and make it seem not as important.

This clarifies the fact that shared references are also `Copy`, by mentioning it as a
separate item in the list of "additional implementors".

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