R1:acfc3c3e5d84

Add an "instructions" key to custom fields

Summary: Ref T1049. I'm planning to use this in Harbormaster custom fields shortly.

Test Plan: {F133843}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8606