Rework & expand discussion: express patent license

Upon consultation with Richard Fontana, we drafted together this rewrite
of the original paragraph discussing this issue.

The original paragraph was tersely written and indeed accurate.
However, it was likely comprehensible only to those already familiar
with patent licensing regimes and systems. As such, it fit poorly in
the tutorial, which is designed for all policy makers who care about
copyleft, who may in fact be new to patent policy and licensing.

Please note: Richard Fontana <fontana@sharpeleven.org> dictated to me
some of this text, and therefore he is likely a copyright and
"creator" (per CC-BY-SA 4.0) of some of this text. In fact, since we
wrote it collaboratively, I suspect Fontana and I are co-copyright
holders and co-creators of this commit.