Fix PhutilMissingSymbolException documentation pointer

Summary: See rPHU91e4f9144e944c2653ce85174abc26099afa61d1. The controlling documentation moved to Phabricator after 'arc liberate', tailor the instructions for modern toolsets / doc locations.

Test Plan: Added "new Derp()" to a script, ran it, got a more useful/actionable error.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2129