message: api: add deprecation warning to comment
  The proto field is marked as deprecated without any explanation, so this
  adds a reference to the other field which should be used instead.
diff: @@ -21,6 +21,8 @@ message HealthCheck {
  // Specifies the incoming HTTP endpoint that should be considered the
  // health check endpoint. For example */healthcheck*.
  + // Note that this field is deprecated in favor of
  + // :ref:`headers <envoy_api_field_config.filter.http.health_check.v2.HealthCheck.headers>`.
  string endpoint = 2 [deprecated = true];
  // If operating in pass through mode, the amount of time in milliseconds
  