Update for new Zend\Http\Header classes

[The section on `Zend\Http\Header` classes](https://github.com/zendframework/zf2-documentation/blob/master/docs/languages/en/modules/zend.http.headers.rst#list-of-http-header-types) needs to be updated to reflect the addition of the `Content-Security-Policy` header class.

Three quick questions before I submit a PR:

1. Do I submit a PR with a base branch of `develop` or `master`?
2. The [`TODO.md`](https://github.com/zendframework/zf2-documentation/blob/master/TODO.md#broken-documentation) file states that the `Zend\Http` documentation is "broken". What is incomplete/incorrect about it?
3. It would be nice to have a cleaner way to document header-specific API. How would this be done? As an example, the unformatted text block that is API documentation for `Set-Cookie` is not super intuitive.