Update nginx configuration documentation.
Summary: The configuration guide for nginx recommends using a conditional statement to check if a file exists, which is listed on the nginx wiki as a common pitfall. It is not necessary to use a conditional and probably should not be recommended to do so.
Test Plan: We use this nginx configuration in our installation.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9401