Update 09.1-animation.md

Add note about ReactTransitionGroup, that any additional, user-defined, properties will be become properties of the rendered component.
And example how to render a `<ul>` with css class.
Fix JSDoc tag in ReactElement.js
Deprecate isValidClass

Fixes #2374
Merge pull request #2381 from shripadk/jsdoc-fix

Fix JSDoc tag in ReactElement.js
Merge pull request #2373 from genbit/docs-animation-props

Update 09.1-animation.md
[Docs] Remove most of @jsx

I kept some places intact (search for @jsx) because they require other bigger changes:

- ref-01-top-level-api.md
- grunt/tasks/npm.js
- old blog posts (don't change those)
- examples/ folder, as they have their own package.json that rely on old dependencies (node-jsx, reactify) that haven't upgraded to 0.12
Merge pull request #2384 from sebmarkbage/deprecateisvalidclass

Deprecate isValidClass
Support but warn on key={null}

Fixes #2386.

Test Plan: jest
Merge pull request #2370 from chenglou/rm-school

[Docs] Remove most of @jsx
Merge pull request #2388 from spicyj/null-key

Support but warn on key={null}
Drop ReactDOM from internal DOM extensions

These used to be exposed as strings on the legacy factories but are now
gone.
Drop Legacy Factories Around Classes

Classes are now pure classes without a legacy factory around them.

Since classes will become just any function that returns a valid instance,
let's drop isValidClass.

There's some hacks in here for auto-mocking frameworks (jest) that mock the
prototype of these classes. These hacks allow these classes to be mounted.