Issue #3389311 by jurgenhaas, kopeboy: Document if ECA Access is required by ECA Enpoint

include/plugins/eca_endpoint/event/eca_endpoint_response.md

+!!! attention
+
+    Don't forget to also add an [ECA Endpoint access](/plugins/eca/endpoint/events/eca_endpoint_access)
+    to your model, so that you can allow this endpoint to be requested, unless
+    you determine that by some other means.
+
+    It's likely that you will need the [Set access result](/plugins/eca/access/actions/eca_access_set_result)
+    for this, which is available from the ECA Access sub-module, that needs to
+    be enabled separately.