Add a note suggesting restarting daemons for feed.http-hooks

Summary:
I was trying to set up a http hook, but despite setting the config,
the endpoint wasn't getting a request. I was advised on IRC by balpert to
restart my daemons and it worked great after I did that.

Since this information isn't in the documentation, I am adding it to the
description of the option, so it helps the next person.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran, spicyj

Differential Revision: https://secure.phabricator.com/D8447