Provide a hint about how to quote search terms containing literal colons

Summary: See https://phabricator.wikimedia.org/T243483. Provide a more direct path forward if users hit the "unknown function" error but are trying to search for a term with a colon in it.

Test Plan:
{F7414068}

{F7414067}

Differential Revision: https://secure.phabricator.com/D21209