R1:ef3df7e4bf5d

Clarify display of Custom Fields

Summary: Fixes T6366, feel free to suggest better copy.

Test Plan: Review copy

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6366

Differential Revision: https://secure.phabricator.com/D10739