[jdom-interest] doc comment for addContent(String)

[philip.nelson at omniresources.com]

> Since this is a change from past behaviour I think you should 
> add a comment
> that calling 
> addContent(null) will now create a new empty content list of 
> the default
> size.

recant!  I think we should test if the text added is null and throw an
exception if it is.  I still don't think the NPE is the best exeception to
throw but that seems to be the consensus for other similar handled
situations.