Added description parameter to the woocommerce_form_field function

With this it is possible to add a short description below every checkout billing and shipping input field.
A sample screenshot can be seen here.
https://www.evernote.com/shard/s3/sh/c5fe354b-5647-4e6a-ba1f-0ce3346b94e7/ffd95aa5196bf4740338a3ac2aa307db