[jdom-interest] Setting / Adding Elements "in between"

[Alex Rosen]

Operations on the returned live list take effect immediately, so there's no
need to call setMixedContent() (which is now called setContent()). The list
returned from getMixedContent()/getContent() is special, and handles setting
and unsetting the parents of added and removed items.

I'll add some JavaDoc comments to Element and Document to make this clearer.

Alex