Improve course review instructions

[philipmjohnson]

Please do work for this task in a branch called issue-782.

We are getting many reviews that get rejected due to the following problems with the comments field:

• Just a couple of words
• Took the class in high school
• Took the class at another university.

We need to enhance the review instructions to clarify that this is not what we're looking for. And make sure that you don't have to review all of your courses to get to Level 6.