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

src/docs/user/configuration/custom_fields.diviner

@@ -101,6 +101,8 @@ When defining custom fields using a configuration option like
   - **default**: Default field value.
   - **strings**: Allows you to override specific strings based on the field
     type. See below.
+  - **instructions**: Optional block of remarkup text which will appear
+    above the control when rendered on the edit view.
 
 The `strings` value supports different strings per control type. They are: