Author: Helen Foster
MDL-21695 adding help string
http://git.moodle.org/gw?p=moodle.git;a=commit;h=b55ff6037885ac6f5546c01c2a35d15bb8e6c62b
http://github.com/moodle/moodle/commit/b55ff6037885ac6f5546c01c2a35d15bb8e6c62b
+ 2.0 en [correctanswerformula,mod_quiz]