Warn users that compound terms separated by apostrophes don't work in the MySQL FULLTEXT index either

Summary: Ref T12928. Like v0.1, terms in the form yo's (sequences of two or fewer characters separated by apostrophes) do not get indexed.

Test Plan: {F5078192}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12928

Differential Revision: https://secure.phabricator.com/D18324