jQuery URL docs is misleading and not aligned with that is actually done

[sevdog]

Currently what is stated in the docs is not aligned with what the code does: https://github.com/jazzband/django-smart-selects/blob/1f3ac7c08a31d2e8cc792f1de484a027a25eef1f/docs/settings.md?plain=1#L3-L10

https://github.com/jazzband/django-smart-selects/blob/1f3ac7c08a31d2e8cc792f1de484a027a25eef1f/smart_selects/widgets.py#L16-L51

Also there are no tests which ensures this beaviour.

The issues are:

• [ ] If JQUERY_URL=True it results in having a True as JS source, which ends up in an errore on frontend
• [ ] It is not documented that to use USE_DJANGO_JQUERY the JQUERY_URL should be a falsy value different from False (ie: empty string, 0, None, etc...)
• [ ] It is wrongly documented that using JQUERY_URL=True will make this package use the Google CDN, while this works if the settings is not defined

Some tests over this feature are required and also the docs should be more accurate.