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.
We might use an alternate phrase rather than "tagged with" as that might lead people to believe that they actually need to apply a tag or label. Do you have a suggestion for where in the comment they put these keywords? Maybe state that in the above paragraph?