Add details about healthcheck_http_method

docs/reference.md

@@ -2004,6 +2004,8 @@ attributes:
 
 `scheme`: HTTP scheme for the service endpoint. The default is *https*.
 
+`healthcheck_http_method`: HTTP method to use for checking job status. The default is *HEAD* to query the endpoint without transferring all the metric data. Not all endpoints support this method, so if the probe fails set it to a method that it does support (worst case: *GET*).
+
 `metrics_path`: Path for metrics if different from the default of `/metrics`.
 
 `labels`: An optional dictionary of key/value labels to set for this