Treat origin as a closed set of supported values

Update the origin parameter description to point at the
Integrations list, and drop the 'Other sources' note that
implied customers could pick any origin string.

Author: claude

docs/configuration/exporting/send-events-api.rst

@@ -50,7 +50,7 @@ Query Parameters
      - Yes
    * - ``origin``
      - string
-     - Identifies the monitoring product (for example ``alertmanager``, ``pagerduty``, ``datadog``).
+     - Identifies the monitoring product. Must be one of the supported origins listed under `Integrations`_ below.
      - Yes
    * - ``account_id``
      - string
@@ -200,7 +200,3 @@ Other
         :link: send-events/solarwinds
         :link-type: doc
 
-Other sources
--------------
-
-Any monitoring system that can POST JSON to a URL is supported. Choose a value for ``origin`` that identifies your source, then paste the URL into your vendor's webhook configuration.