Show the default value in Config.
Summary:
As mentioned by @epriestley in an inline on D4290, we should show what happens
if the user leaves the box blank.
Test Plan: Went to edit a setting and saw the default below the text box.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, asherkin
Differential Revision: https://secure.phabricator.com/D4293