Added a comment to explain a change I made in Issue 958

Fixed: Issue 965 - Change made to /mysite/static/css/missions/base.css in Issue 958 needs some explanation. <https://openhatch.org/bugs/issue965>
Updated the Front-end style guide accordingly. <https://openhatch.org/wiki/Front-end_style_guide#Editing_the_training_mission_hints>