Add doc for new Show Reset Button setting

Modified Creating Course Content > Working with Problem Components: added doc for the new Show Reset Button setting (DOC-1140)
https://github.com/edx/edx-platform/pull/5595
@mhoeber @srpearce @lamagnifica @sarina @explorerleslie Please review
could be in follow on PR, but I'd add a screen shot of student view in LMS with a problem with reset button