Restore a 'however'

This 'however' was changed to 'additionally' in #5, suggested by @mwweinberg, to avoid implying a contradiction that wasn't there, but I think we all might have read it too fast. The sentence it begins is actually in contrast to the previous sentence, because we're talking about works that have a statutory exemption *from* the exemption from copyright (and have been granted US copyrightability).

I'm proposing this as a merge into `v3`, as I don't want to disturb the doc, and it's not a really important change (doesn't change the substantive text at all) and can wait as long as is needed.