Allow [[]] remarkup links outside Phabricator

Summary: Namely at /docs/article/Give_Feedback!_Get_Support!.html.

Test Plan:

[[wiki]]
[[http://example.com]]
[[http://example.com | example.com]]

Regenerate docs.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2110