The `React.createComponent` method is being deprecated in favor of
`createReactComponent`.

It no longer makes sense to have a section for the 'createClass' method
in this page, since it won't be available as a top level method on
'React'.

I initially was going to pull the section about 'createClass' into a
separate page to add under 'addons' but it was short and duplicative of
the 'react-without-es6' docs. So I just linked to those.

I am doing the docs for `context` in a separate commit because that case
was a bit less clear-cut.

We will no longer support `React.PropTypes` as a built-in feature of
React, and instead should direct folks to use the `PropTypes` project
that stands alone.

Rather than retaining the `React.PropTypes` examples and just revamping
them to show the use of the stand-alone `PropTypes` library with React,
it makes more sense to direct people to that project and reduce the
perceived API area and complexity of React core. The proper place to
document `PropTypes` is in the README or docs of that project, not in
React docs.

We use `React.PropTypes` to define the `contextType` for the `context`
feature of React. It's unclear how this will work once `React.PropTypes`
is replaced by the external `PropTypes` library. Some options;

a) Deprecate `context`, either in v16 or shortly after. Seems reasonable
based on the intense warnings against using context that we have in the
docs -
https://facebook.github.io/react/docs/context.html#why-not-to-use-context
**Except** that probably some widely used libraries depend on it, like
`React-Router`.

b) Expect users will use external `PropTypes` library when defining
`contextTypes` and just don't do our `checkReactTypeSpec` against them
any more in v16.

c) Stop masking context and pass the whole context
unmasked everywhere. Worst option, do not recommend.

I went with `b` and assume that, for now, we will get users to use the
external `PropTypes` when defining context. I will update this PR if we
want a different approach.