Update configuration instructions for Apache 2.4
Fixes T4061. Following the instructions in the documentation with Apache 2.4 (which is installed
with Ubuntu 13.10 and other distributions) will result in a "403 forbidden" error.
The instruction provides information on how to fix it.
Test Plan: Tested on apache 2.4 install
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran, asherkin
Maniphest Tasks: T4061
Differential Revision: https://secure.phabricator.com/D7529