Since the trashpurge.php script has been backported to previous versions it makes sense to have it documented, as no documentation exists for this script so far, apart from the script's --help.

Here's the issue related to the backport:
http://issues.ez.no/IssueView.php?Id=10827&

The script should be included in the following list:
http://doc.ez.no/eZ-Publish/Technical-manual/4.x/Reference/Scripts/Generic

One details that should be included in the document is that, since this script is intended to clean the trash bin we don't find it necessary to specify a siteaccess (using the -s parameter) for the script to get the settings.
However, if we don't specify a siteaccess the default one will be used, and only objects translated in the languages included in the default siteaccess will be purged from the trash bin. This means that only some part of the trash bin's contents will be purged.

So, the recommended usage is:

php bin/php/trashpurge.php -s <ADMIN_SITEACCESS>

This gives us some more flexible trash purging options, if we want to purge trashed content of a given language only, or trashed content created from a specific siteaccess.
In that case, the correct usage will be:

php bin/php/trashpurge.php -s <DESIRED_SITEACCESS>