Describe Rails subscriber implementation

The agent uses a non-traditional way to subscribe to Rails events.
Instead of passing a block to ActiveSupport::Notifications.subscribe, the
agent defines a #start and #finish method, which Rails responds to.
This is due to a threading issue discovered on initial instrumentation.
Issue: rails/rails#12069
Rails code: https://github.com/rails/rails/blob/ed5af004598fa7645fec2210453cca00f4b59168/activesupport/lib/active_support/notifications/fanout.rb#L320

lib/new_relic/agent/instrumentation/notifications_subscriber.rb

@@ -47,6 +47,9 @@ def self.subscribe(pattern)
           end
         end
 
+        # The agent doesn't use the traditional ActiveSupport::Notifications.subscribe
+        # pattern due to threading issues discovered on initial instrumentation.
+        # Instead we define a #start and #finish method, which Rails responds to.
         def start(name, id, payload)
           return unless state.is_execution_traced?