docs: net: add missing devlink health cmd - trigger

Documentation is missing and it's not very clear what
this callback is for - presumably testing the recovery?

Signed-off-by: Jakub Kicinski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/devlink/devlink-health.rst

···
      - Allows reporter-related configuration setting.
    * - ``DEVLINK_CMD_HEALTH_REPORTER_RECOVER``
      - Triggers reporter's recovery procedure.
+   * - ``DEVLINK_CMD_HEALTH_REPORTER_TEST``
+     - Triggers a fake health event on the reporter. The effects of the test
+       event in terms of recovery flow should follow closely that of a real
+       event.
    * - ``DEVLINK_CMD_HEALTH_REPORTER_DIAGNOSE``
      - Retrieves current device state related to the reporter.
    * - ``DEVLINK_CMD_HEALTH_REPORTER_DUMP_GET``