Action Cable assert_broadcasts API docs [ci skip]

Follow-up to [rails#48798][]
Reverts [rails#47837][]

The `capture_broadcasts` helper was introduced in [rails#48798][] to remove
the return value from the `assert_broadcasts` call. The behavior was
changed, but the corresponding documentation for the `assert_broadcasts`
method did not revert the changes added in [rails#47837][].

This change should probably also get backported to `7-1-0-stable`.

[rails#47837]: rails#47837
[rails#48798]: rails#48798

Co-authored-by: Matheus Richard <[email protected]>

Author: seanpdoyle

actioncable/lib/action_cable/test_helper.rb

@@ -32,21 +32,17 @@ def after_teardown # :nodoc:
     #     end
     #
     # If a block is passed, that block should cause the specified number of messages
-    # to be broadcasted. It returns the messages that were broadcasted.
+    # to be broadcasted.
     #
     #     def test_broadcasts_again
-    #       message = assert_broadcasts('messages', 1) do
+    #       assert_broadcasts('messages', 1) do
     #         ActionCable.server.broadcast 'messages', { text: 'hello' }
     #       end
-    #       assert_equal({ text: 'hello' }, message)
     #
-    #       messages = assert_broadcasts('messages', 2) do
+    #       assert_broadcasts('messages', 2) do
     #         ActionCable.server.broadcast 'messages', { text: 'hi' }
     #         ActionCable.server.broadcast 'messages', { text: 'how are you?' }
     #       end
-    #       assert_equal 2, messages.length
-    #       assert_equal({ text: 'hi' }, messages.first)
-    #       assert_equal({ text: 'how are you?' }, messages.last)
     #     end
     #
     def assert_broadcasts(stream, number, &block)