Add contextual typeahead function documentation

Summary: Ref T4100. This integration into the "Browse" dialog is probably a little more heavy-handed than we should shoot for.

Test Plan:
{F377131}

{F377132}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12482