Add docs regarding anything inside `<html>` within a template that's not inside `<head>` or `<body>` will get stripped

This is due to generally ignoring the <html> element's inner content, and building it based on the JSDOM tree manually. It has to be done manually, because `<head>` and `<body>` should only be included if the source included them (but JSDOM automatically generates a "good" DOM tree, so the JSDOM instance of the page includes those pieces).