docs: update customization UI hints

fern/docs/pages/customization.mdx

 "created_by": {...}, 
 This guide provides an overview of the customization framework and walks you through the process of tracking bugs in your organization. By the end of this guide, you'll be able to: 
 "created_by": {...}, 
@@ -390,6 +390,10 @@ hints:
 * `placeholder`: The placeholder text for the field.
 * `is_sortable`: Whether the field is sortable. Requires `is_filterable` to be true.
 * `is_groupable`: Whether the field is groupable. Requires `is_filterable` to be true.
+* `order`: The order in which the field appears in the side panel.
+* `group_name`: The group title under which field(s) appear in the side panel. In the example below, the fields are grouped under groups titled **Group 1** and **Group 2**.
+
+  ![customization-group_name-ui-hint](../img/customization-group_name-ui-hint.png)
 
 <Callout intent="tip">
 `is_filterable` is not a UI hint but a top level field property.
@@ -651,3 +655,7 @@ The `effects` array contains the list of effects of the condition. The following
 `don:core:dvrv-us-1:devo/test:stage/5` is the ID of the _completed_ stage. The
 stage display name is not used in the expression because it is liable to change.
 </Callout>
+
+Constraints on stock fields can only be strengthened.
+For example, a required stock field cannot be made optional, and a hidden stock
+field cannot be made visible.