Fix typos and errors in documentation.

Besides typos, I also changed the InitializingScoreTrend explanation, which I believe to be explained incorrectly. I discussed this on the forum, but never got a reply, see https://groups.google.com/forum/#!topic/optaplanner-dev/diEMZW4jQ0E.
should actually be "cannot unmatch a negative constraint"
I 'll rewrite this part, as it's unclear (both the original and the changed version aren't really correct)