Add a line to describe the new Chef provisioner option (#17640)

website/docs/provisioners/chef.html.markdown

@@ -65,6 +65,9 @@ The following arguments are supported:
   for the new node. These can also be loaded from a file on disk using the [`file()`
   interpolation function](/docs/configuration/interpolation.html#file_path_).
 
+* `channel (string)` - (Optional) The Chef Client release channel to install from. If not
+  set, the `stable` channel will be used.
+
 * `client_options (array)` - (Optional) A list of optional Chef Client configuration
   options. See the [Chef Client ](https://docs.chef.io/config_rb_client.html) documentation
   for all available options.