Add comment to ReactiveCaching module of new docs

The comment in this module was added to our utilities documentation in
https://gitlab.com/gitlab-org/gitlab/merge_requests/19129.

This change updates the comment to refer to the new documentation page.

https://gitlab.com/gitlab-org/gitlab/issues/35748

app/models/concerns/reactive_caching.rb

 # frozen_string_literal: true
 
-# The ReactiveCaching concern is used to fetch some data in the background and
-# store it in the Rails cache, keeping it up-to-date for as long as it is being
-# requested.  If the data hasn't been requested for +reactive_cache_lifetime+,
-# it stop being refreshed, and then be removed.
-#
-# Example of use:
-#
-#    class Foo < ApplicationRecord
-#      include ReactiveCaching
-#
-#      after_save :clear_reactive_cache!
-#
-#      def calculate_reactive_cache
-#        # Expensive operation here. The return value of this method is cached
-#      end
-#
-#      def result
-#        with_reactive_cache do |data|
-#          # ...
-#        end
-#      end
-#    end
-#
-# In this example, the first time `#result` is called, it will return `nil`.
-# However, it will enqueue a background worker to call `#calculate_reactive_cache`
-# and set an initial cache lifetime of ten minutes.
-#
-# The background worker needs to find or generate the object on which
-# `with_reactive_cache` was called.
-# The default behaviour can be overridden by defining a custom
-# `reactive_cache_worker_finder`.
-# Otherwise the background worker will use the class name and primary key to get
-# the object using the ActiveRecord find_by method.
-#
-#    class Bar
-#      include ReactiveCaching
-#
-#      self.reactive_cache_key = ->() { ["bar", "thing"] }
-#      self.reactive_cache_worker_finder = ->(_id, *args) { from_cache(*args) }
-#
-#      def self.from_cache(var1, var2)
-#        # This method will be called by the background worker with "bar1" and
-#        # "bar2" as arguments.
-#        new(var1, var2)
-#      end
-#
-#      def initialize(var1, var2)
-#        # ...
-#      end
-#
-#      def calculate_reactive_cache
-#        # Expensive operation here. The return value of this method is cached
-#      end
-#
-#      def result
-#        with_reactive_cache("bar1", "bar2") do |data|
-#          # ...
-#        end
-#      end
-#    end
-#
-# Each time the background job completes, it stores the return value of
-# `#calculate_reactive_cache`. It is also re-enqueued to run again after
-# `reactive_cache_refresh_interval`, so keeping the stored value up to date.
-# Calculations are never run concurrently.
-#
-# Calling `#result` while a value is in the cache will call the block given to
-# `#with_reactive_cache`, yielding the cached value. It will also extend the
-# lifetime by `reactive_cache_lifetime`.
-#
-# Once the lifetime has expired, no more background jobs will be enqueued and
-# calling `#result` will again return `nil` - starting the process all over
-# again
+# The usage of the ReactiveCaching module is documented here:
+# https://docs.gitlab.com/ee/development/utilities.html#reactivecaching
 module ReactiveCaching
   extend ActiveSupport::Concern