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