R1:a9f38e55e5e4

Modernize Facebook OAuth instructions

Summary: Ref T4593. We have somewhat outdated instructions, and safer setup instructions are now available.

Test Plan: Read instructions.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4593

Differential Revision: https://secure.phabricator.com/D8518