I disagree with the other answer here; write whatever you want and as much as you want.  If your professor refuses to communicate allowable scope or restrictions then there are none.  He cannot reasonably hold you to restrictions that exist only in his imagination.  (Also, keep his email in case there is a dispute over this.)

------------------

**Update:** Additional information has been added to the question after my answer, showing the actual email lines the professor sent.  In my view, his statement could be taken to imply that he is one of the professors who wants to you restrict your sheet to only formulas (i.e., no accompanying textual explanation) and so he *has* elaborated on what he means (contra the original question).  Since many people already voted on the original question and answer I am posting this as an update rather than revising my original answer.