Update docs for ActiveRecord::Middleware::DatabaseSelector [ci skip]

The docs for this middleware indicate that you can set a delay that
can be a duration (eg. `2.seconds`).

https://api.rubyonrails.org/v6.1/classes/ActiveRecord/Middleware/DatabaseSelector.html

In normal apps, this works fine without an explicit require because
the extension is required through other libs. However, when creating a
minimal app, this results in an undefined method error.

This is something that is already being included in
test/development/production config files since c8c5497.

activerecord/lib/active_record/middleware/database_selector.rb

@@ -22,9 +22,13 @@ module ActiveRecord
     # To use the DatabaseSelector in your application with default settings add
     # the following options to your environment config:
     #
-    #   config.active_record.database_selector = { delay: 2.seconds }
-    #   config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
-    #   config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
+    #   require "active_support/core_ext/integer/time"
+    #
+    #   class Application < Rails::Application
+    #     config.active_record.database_selector = { delay: 2.seconds }
+    #     config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
+    #     config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
+    #   end
     #
     # New applications will include these lines commented out in the production.rb.
     #