With Comments Extension, you can add help text to every single input in the WordPress Comment form or change the default Labels displayed within the form inputs.
Default Comment Form inputs are Comment, Name, Email, and Website. For all of the mentioned inputs you can change the default label text and add an additional message which is displayed below the input field like on the following image:
Disabling URL in comments
If you want to take control of all links going out from your website or blog, this setting is pretty useful.