Add docs for agent policy custom fields

docs/en/ingest-management/agent-policies.asciidoc

@@ -271,7 +271,9 @@ Any agents assigned to a policy must be unenrolled or assigned to a different po
 [[add-custom-fields]]
 == Add custom fields
 
-Use this setting to add a custom field and value set to all data collected from the {agents} enrolled in this policy.
+Use this setting to add a custom field and value set to all data collected from the {agents} enrolled in an {agent} policy. This is useful when you want to identify or visualize all of the data from a group of agents, and possibly manipulate it downstream.
+
+To add a custom field:
 
 . In {fleet}, click **Agent policies**.
 Select the name of the policy you want to edit.
@@ -280,13 +282,29 @@ Select the name of the policy you want to edit.
 
 . Click **Add field**.
 
+. Specify a field name and value.
++
+[role="screenshot"]
+image::images/agent-policy-custom-field.png[Sceen capture showing the UI to add a custom field and value]
+
+. Click **Add another field** for additional fields, or click **Save changes** when you're done.
+
+To edit a custom field:
+
+. In {fleet}, click **Agent policies**.
+Select the name of the policy you want to edit.
+
+. Click the **Settings** tab and scroll to **Custom fields**. Any custom fields that have been configured are shown.
+
+. Click the edit icon to update a field or click the delete icon to remove it.
+
 Note that for a small set of inputs, adding custom tags is not supported:
 
 * `apm`
 * `cloudbeat` and all `cloudbeat/*` inputs
 * `cloud-defend`
 * `fleet-server`
-* `pf-host-agent`, `pf-elastic-symbolizer`, `pf-elastic-collector`
+* `pf-elastic-collector`, `pf-elastic-symbolizer`, and `pf-host-agent`
 
 
 [discrete]