Clarify how to configure OAuth, particularly for GitHub

Summary:
GitHub needs an unguessably specific URI in its configuration for OAuth.

Test Plan:
Regenerated documentation and read it over. (These instructions are based on the
existing applications, which appear to function correctly.)

Reviewed By: j3kuntz
Reviewers: jungejason, aran, tuomaspelkonen, j3kuntz
CC: aran, epriestley, j3kuntz
Differential Revision: 307