Document how to use the disable field + example

There's some instructions here:

https://www.drupal.org/project/eu_cookie_compliance/issues/3254826 🐛 Disabling Javascript with "Opt-in with categories" enable seems to not work Postponed: needs info

Basically you need to examine files and libraries from the module that you want to prevent loading scripts, and use the file paths that you can find in the page's source, excluding the leading slash.