old pr 271 comment fixups

Most of https://edx-wiki.atlassian.net/browse/STUD-487 afaik. Left 2 less trivial items (factories, writeonce)

@cahrens @chrisndodge Please review: note the __init__ below just moved to top of class def.

@cpennington these address your comments on Locators from the original split mongo merge but I'm not asking you to review b/c you seem backlogged.