Add more details about applying classNames

Author: silvenon

src/CSSTransition.js

@@ -238,13 +238,23 @@ CSSTransition.propTypes = {
    * - `fade-enter`, `fade-enter-active`, `fade-enter-done`
    * - `fade-exit`, `fade-exit-active`, `fade-exit-done`
    *
-   * **Note**: `fade-appear-done` and `fade-enter-done` will _both_ be applied.
-   * This allows you to define different behavior for when appearing is done and
-   * when regular entering is done, using selectors like
-   * `.fade-enter-done:not(.fade-appear-done)`. For example, you could apply an
-   * epic entrance animation when element first appears in the DOM using
-   * [Animate.css](https://daneden.github.io/animate.css/). Otherwise you can
-   * simply use `fade-enter-done` for defining both cases.
+   * A few details to note about how these classes are applied:
+   *
+   * 1. They are _joined_ with the ones that are already defined on the child
+   *    component, so if you want to add some base styles, you can use
+   *    `className` without worrying that it will be overridden.
+   *
+   * 2. If the transition component mounts with `in={false}`, no classes are
+   *    applied yet. You might be expecting `*-exit-done`, but if you think
+   *    about it, a component cannot finish exiting if it hasn't entered yet.
+   *
+   * 2. `fade-appear-done` and `fade-enter-done` will _both_ be applied. This
+   *    allows you to define different behavior for when appearing is done and
+   *    when regular entering is done, using selectors like
+   *    `.fade-enter-done:not(.fade-appear-done)`. For example, you could apply
+   *    an epic entrance animation when element first appears in the DOM using
+   *    [Animate.css](https://daneden.github.io/animate.css/). Otherwise you can
+   *    simply use `fade-enter-done` for defining both cases.
    *
    * Each individual classNames can also be specified independently like:
    *