Common terminology for review comments

@singingwolfboy @sarina @nedbat Please take a look at the suggested terminology and definition proposed in this PR.  I spoke to @singingwolfboy about the benefits of having a common vocabulary for this and he proposed creating this PR to discuss it.  I'd appreciate your feedback.  I find this will be useful within our T&L team and possibly for edX at large.
@cahrens FYI.
I think the word "petty" might have too pejorative a connotation. Maybe "small" or "minor" instead?