Update from 6.x to 7.x: Documentation on theming does not adequately explain what 'render element' is or how you use it.

It took me a long time of looking at code to understand what this meant. The documentation itself glosses over it, and nothing really talks about it on that page, but 'render element' is wholly new and should not be simply glossed over. At worst it should contain a link to some place that explains what that means or how it works. At best it should include a paragraph describing it.