Indicated that change to Puzzle.helpImage is optional.
Indicated that change to Puzzle.helpImage is optional.

Review URL: https://codereview.appspot.com/9127044

git-svn-id: 2cf55447cea94972d775c94cce66c944ede0459c@814 c400ca83-b69d-9dd7-9705-49c6b8615e23

Khan/blockly,tjpalmer/blockly,tobyk100/blockly-trunk,tobyk100/blockly-trunk,tjpalmer/blockly,Khan/blockly,tobyk100/blockly-trunk,tobyk100/blockly-trunk