Bring back documentation on Link DOM props (#5799)
Bring back documentation on Link DOM props (#5799)

In v3 the docs [clearly indicated](https://github.com/ReactTraining/react-router/blob/07d6b50ab9a9b6d1940d67abe530044be7f7dc99/docs/API.md#others) that `<Link>` takes DOM props to be put on `<a>`.
In v4 docs this information [is missing](https://reacttraining.com/react-router/web/api/Link).

Nevertheless, the functionality [is still here](https://github.com/ReactTraining/react-router/blob/96410c69be1ca32e294bf83e7d1321c39ecd286b/packages/react-router-dom/modules/Link.js#L76).
The contributors [refer to it](https://github.com/ReactTraining/react-router/issues/5760#issuecomment-347979291) as expected, public functionality.
The [`activeClassName` prop on `NavLink`](https://reacttraining.com/react-router/web/api/NavLink/activeClassName-string) clearly suggests that a regular `className` is also accepted.
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