Clarify that Threema Gateway Alerts support only Basic IDs (#34828)

Threema Gateway supports two types of IDs: Basic IDs (where the
encryption is managed by the API server) and End-to-End IDs (where the
keys are managed by the user).

This plugin currently does not support End-to-End IDs (since it's much
more complex to implement, because the encryption needs to happen
locally). Add a few clarifications to the UI.

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/tests/api/alerting/api_available_channel_test.go

@@ -1442,14 +1442,14 @@ var expAvailableChannelJsonOutput = `
     "type": "threema",
     "name": "Threema Gateway",
     "heading": "Threema Gateway settings",
-    "description": "Sends notifications to Threema using the Threema Gateway",
+    "description": "Sends notifications to Threema using Threema Gateway (Basic IDs)",
     "info": "Notifications can be configured for any Threema Gateway ID of type \"Basic\". End-to-End IDs are not currently supported.The Threema Gateway ID can be set up at https://gateway.threema.ch/.",
     "options": [
       {
         "element": "input",
         "inputType": "text",
         "label": "Gateway ID",
-        "description": "Your 8 character Threema Gateway ID (starting with a *).",
+        "description": "Your 8 character Threema Gateway Basic ID (starting with a *).",
         "placeholder": "*3MAGWID",
         "propertyName": "gateway_id",
         "selectOptions": null,