Add Grape.start documentation to API docs

Author: estolfo

docs/api.asciidoc

@@ -193,16 +193,27 @@ Returns the built context.
 
 [float]
 [[rails-start]]
-=== Manually hooking into Rails
+=== Rails
 
-Start the agent and hook into Rails explicitly. This is useful if you skip requiring
+Start the agent and hook into Rails manually. This is useful if you skip requiring
 the gem and using the `Railtie`.
 
 [source,ruby]
 ----
 ElasticAPM::Rails.start(server_url: 'http://localhost:8200')
 ----
 
+[float]
+[[grape-start]]
+=== Grape
+
+Start the agent and hook into Grape.
+
+[source,ruby]
+----
+ElasticAPM::Grape.start(MyGrapeApp, server_url: 'http://localhost:8200')
+----
+
 [float]
 === Errors