Improve tooltip.
Improve tooltip.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/9575044

git-svn-id: 2cf55447cea94972d775c94cce66c944ede0459c@923 c400ca83-b69d-9dd7-9705-49c6b8615e23

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