Docs: Mention for `in` prop that it has no effect if used with `TransitionGroup`

[andre-brdoch]

Would be great to mention under the in prop documentation of Transition, that setting it is not needed if it is used together with TransitionGroup.

<TransitionGroup component="ul">
    {/* setting the in prop is useless, since TransitionGroup already controls the value */}
    <CSSTransition in={active} timeout={1000}>
        {(state) => (
            <div>
                <p>State: {state}</p>
                <p>Active: {`${active}`}</p>
            </div>
        )}
    </CSSTransition>
</TransitionGroup>

I would have made a PR myself, but I wasn't sure where to find the code for the docs 🙏