Added documentation for instance headers destroy API

doc/administration/audit_event_streaming/graphql_api.md

@@ -481,6 +481,20 @@ Streaming destination is deleted if:
 - The returned `errors` object is empty.
 - The API responds with `200 OK`.
 
+To remove an HTTP header, use the GraphQL `auditEventsStreamingInstanceHeadersDestroy` mutation. 
+To retrieve the header ID,
+[list all the custom HTTP headers](#list-streaming-destinations) for the instance.
+
+```graphql
+mutation {
+  auditEventsStreamingInstanceHeadersDestroy(input: { headerId: "gid://gitlab/AuditEvents::Streaming::InstanceHeader/<id>" }) {
+    errors
+  }
+}
+```
+
+The header is deleted if the returned `errors` object is empty.
+
 ### Google Cloud Logging configurations
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/409422) in GitLab 16.1.