Correct documentation of config options order

Author: estolfo

docs/configuration.asciidoc

@@ -21,8 +21,9 @@ secret_token: <%= ENV["VERY_SECRET_TOKEN"] %>
 
 Options are applied in the following order (last one wins):
 
+1. Arguments to `ElasticAPM::<framework>.start`
 1. Defaults
-2. Arguments to `ElasticAPM.start` / `ElasticAPM::Grape.start` / `ElasticAPM::Rails.start` / `Config.new`
+2. Arguments to `ElasticAPM.start` / `Config.new`
 3. Config file eg. `config/elastic_apm.yml`
 4. Environment variables
 

lib/elastic_apm/grape.rb

@@ -13,7 +13,7 @@ module Grape
     # @param app [Grape::API] A Grape app.
     # @param config [Config, Hash] An instance of Config or a Hash config.
     # @return [true, nil] true if the agent was started, nil otherwise.
-    def start(app, config)
+    def start(app, config = {})
       config = Config.new(config) unless config.is_a?(Config)
       configure_app(app, config)