Changing position: absolute's description.

Clarified and removed these incorrect statements from the `position: absolute` description:

* elements absolutely positioned are positioned relative to the `<html>` element, not the `<body>` element. [See this jsFiddle for an example.](http://jsfiddle.net/KdZBX/)
* elements absolutely positioned _do not_ scroll with the page. This is a property unique to `fixed`.

I also added info about the fact that `absolute` elements are "popped out" of the DOM, and I moved `position.absolute.2` to `position.absolute.3` to make room for the second paragraph of the description. If adding a new paragraph is troublesome, I'm open to consolidating these two new paragraphs.