: global: reoroder precedences Env > ClientOverride, fix doc inconsistencies (#2182)

Summary:

key change here is precedence: Env wins over ClientOverride. everything else is cleanup to make the code and docs say the same thing and to reduce the chance we "remember" the wrong order later. tests are updated to encode the new behavior.

Reviewed By: mariusae

Differential Revision: D89551496

Author: shayne-fletcher