In #34709 we updated the guide, but `actioncable/README.md` is still
outdated. Instead of fixing content in the file. I suggest
not duplicate the info that is already in the guide and instead remove
the info from the file and just add a message:
"You can read more about Action Cable in the
[Action Cable Overview](https://edgeguides.rubyonrails.org/action_cable_overview.html) guide."

The same approach is being used for Action Mailbox and Action Text,
see #34812 and #34878.