Document that hypothetical requests aren't helpful
Summary: Fixes T8908. We sometimes receive hypothetical requests (usually when existing companies are thinking about adopting Phabricator) in the form of "No users have actually done this yet, but bad thing Y might happen". We can almost never move forward with these. Explain this in detail.
Test Plan: Reading.
Reviewers: btrahan, chad
Reviewed By: chad
Maniphest Tasks: T8908
Differential Revision: https://secure.phabricator.com/D13663