Update documentation for "uri.allowed-protocols"

Summary: See https://discourse.phabricator-community.org/t/download-tasks-and-others-as-excel-throw-exception/.

Test Plan: Read config.

Reviewers: amckinley, avivey

Reviewed By: avivey

Subscribers: avivey

Differential Revision: https://secure.phabricator.com/D20430