Provide a setup warning about ft_min_word_len
Summary: Fixes T4130. Adds a setup warning when ft_min_word_len is set to the default value.
Test Plan: Hit setup warning; resovled setup warning. Searched for "DOS".
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4130
Differential Revision: https://secure.phabricator.com/D10259