Audit terraform variable descriptions

[triceam]

Tech Debt
Many of our terraform modules have vague, incomplete, or sometimes incorrect descriptions for input variables. It would be worthwhile to audit or modules (starting with infrastructure) to make sure they have valid and applicable descriptions for all intputs/outputs, so users that are unfamiliar with the modules have a better idea what they're supposed to do.

[thomassuedbroecker]

We should flag the effected variables in modules with a comment here and add a link to the related module.
@triceam @seansund