Document AC::Connection::Authorization#reject_unauthorized_connection

This method is repeatedly used throughout the docs (in the [AC::Connection docs](12b6849858/actioncable/lib/action_cable/connection/base.rb (L28)), the [AC README](12b6849858/actioncable/README.md (a-full-stack-example)), the [AC Guides](12b6849858/guides/source/action_cable_overview.md (connection-setup))), but not actually documented itself and seemingly not supported for public use based on its current `private` status.

This actually makes the method public and documents it.  The actual behavior that’s documented here is implemented [here](12b6849858/actioncable/lib/action_cable/connection/base.rb (L213-L219)), via [this rescuing of the UnauthorizedError](3dd1de8ba4/actioncable/lib/action_cable/connection/base.rb (L172)).

The method is [already tested here](25473baf40/actioncable/test/connection/authorization_test.rb (L17-L29)).

actioncable/lib/action_cable/connection/authorization.rb

@@ -3,11 +3,11 @@ module ActionCable
     module Authorization
       class UnauthorizedError < StandardError; end
 
-      private
-        def reject_unauthorized_connection
-          logger.error "An unauthorized connection attempt was rejected"
-          raise UnauthorizedError
-        end
+      # Closes the \WebSocket connection if it is open and returns a 404 "File not Found" response.
+      def reject_unauthorized_connection
+        logger.error "An unauthorized connection attempt was rejected"
+        raise UnauthorizedError
+      end
     end
   end
 end