When sentences get this long we like to change the T() call where the first argument is a very short, pithy summary of the string. We do this because the English is likely to change in longer sentences and we don't want that to fully break translations. Do something like this:

T('Add a white-list of trusted domains.', 'You can optionally specify a white-list of trusted domains (ie. yourdomain.com) that are allowed to embed elements of your community (forum, comments, or modules).')