Add some documentation for constants

[qwergle]

Here's the documentation I've written for the constants. Some other minor changes have also been added, such as updating the Credits section, adding notes, and fixing typos.

Note to @AlexAltea: I likely won't be able to perform any revisions you ask of me, due to the two of us being in different time zones (I was asleep when you asked me to remove the Helpers section).

[qwergle]

I'm happy to help (plus, writing documentation gives me something to do)!