Follow-up from "Add ReactiveCaching to developer docs"

The following discussion from !19129 (merged) should be addressed:

• @seanarnold started a discussion: (+1 comment)

  @.luke Do you think it'd be useful to add a note in reactive_caching.rb to let readers know that this new documentation has been written?

  There will be a few of us who are used to the old in-code docs and might not think to update utilities.md.