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
I'd take out as much passive voice here as possible:

This button clears the student input so that the student can try to answer the problem again. If the problem contains randomized variables and the randomization setting is **On Reset**, the student sees a different version of the problem.