R1:7bee68a763bd

Document configuration of external editor links

Summary: Provide some documentation for this feature since it's not super
obvious how it works.

Test Plan: Generated documentation, read documentation.

Reviewers: btrahan, vrana, jungejason, nh

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1521