[ci skip] Link to the request helpers documentation.

It's tough for people without the knowledge of where the `get` and
friends integration test helpers are defined to find documentation
for them. Add a link to the main integration test documentation.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -667,6 +667,9 @@ module ActionDispatch
   #       end
   #   end
   #
+  # See the {request helpers documentation}[rdoc-ref:ActionDispatch::Integration::RequestHelpers] for help on how to
+  # use +get+, etc.
+  #
   # === Changing the request encoding
   #
   # You can also test your JSON API easily by setting what the request should