Comment about the importance of script defer

See [this epic][1].

[1]: https://gitlab.com/groups/gitlab-org/-/epics/4538#note_432159769

app/helpers/defer_script_tag_helper.rb

 # frozen_string_literal: true
 
 module DeferScriptTagHelper
-  # Override the default ActionView `javascript_include_tag` helper to support page specific deferred loading
+  # Override the default ActionView `javascript_include_tag` helper to support page specific deferred loading.
+  # PLEASE NOTE: `defer` is also critical so that we don't run JavaScript entrypoints before the DOM is ready.
+  # Please see https://gitlab.com/groups/gitlab-org/-/epics/4538#note_432159769.
   def javascript_include_tag(*sources)
     super(*sources, defer: true)
   end