Document the behaviour of unfiltered_html for lower level users

If an Administrator or Editor places some JavaScript into the content of a post and then adds an Author or Contributor as an attributed author, the JavaScript will be stripped out if the Author or Contributor make an edit to the post. This is due to the `unfiltered_html` capability.

We mustn't change this functionality, but it ought to be documented so users are clear about what happens in this situation.