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

Subscribers: epriestley

Maniphest Tasks: T8908

Differential Revision: https://secure.phabricator.com/D13663