Hi Bernd,
Thank you for the feedback. I’ll add more descriptions for the next update.
Nofollow is a common tag that you would add to your links for SEO reasons, if you aren’t sure what it is then there is no need to worry and you can leave it a lone – https://support.google.com/webmasters/answer/96569?hl=en
Correct, if the URL is empty then the button won’t be added neither for display or in the code. The button is only added when a valid URL is entered.
– AJ
All right, thanks. Still would appreciate a description within the Customizer, though.