Clarify animating one or zero items in docs

When I first read these docs, it was not immediately clear to me that I could
use `React.addons.CSSTransitionGroup` to animate a single item coming into view,
or an item replacing an item already there. This was partly due to the example
which rendered a list of items.

This PR adds a blurb about being able to use
`React.addons.CSSTransitionGroup` with one or zero items, provides a code
example, and adds a note blockquote that a `key` attribute must always be
provided on each child of `React.addons.CSSTransitionGroup`. The latter point
was not immediately obvious from the original `TodoList` code example, since it
renders a list which would normally require `key` attributes anyway.

Test Plan:
- Refreshed `http://localhost:4000/react/docs/animation.html`, saw that the
  docs additions rendered correctly.
- Example code not tested (it was extracted from working code).

(cherry picked from commit 721f3973037c8650b45a479c24c94e2bf7b9e22e)