Carol/docs/doc 1199

@ahodges @srpearce @mhoeber 
Updates to correct the usage of "register" vs "enroll" for courses.
@srpearce I don't think "Step X" with cap S is awful. If you agree, for this story I'm going to leave these instances as is; this was in existing doc and the usage might extend beyond this file. Would rather fix all at once in future rather than risk creating some ugly inconsistencies.