Tabulate EA/EH environment variables (Azure#3524)

Tabulating all the known settings knobs via environment variables for official documentation.  I tried to track down everything, but the config readers are kinda spread out everywhere, so I might have missed a few.

I've also taken the liberty to mark some of these as omitted/obsolete if I don't think they should be customer facing.  Lemme know if you'd like any of those included, otherwise I'll have them removed from this doc before merging.