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
Before/as part of the 2nd sentence, identify what value you have to specify for the rest of this paragraph to be correct.