Explain upstream attitudes toward CLI exit codes

Summary: Ref T5991. See D14116. We are consistent but nonstandard in our use of exit codes. This document explains what we use exit codes for and why we do this.

Test Plan: Read it.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5991

Differential Revision: https://secure.phabricator.com/D14173