I unfortunately updated already and they seem to have added documentation into man systemd-tmpfiles already. Here is the snippet that is relevant:
<span style="color:#323232;">--purge
</span><span style="color:#323232;">If this option is passed, all files and directories marked for creation by the tmpfiles.d/ files specified on the command
</span><span style="color:#323232;">line will be deleted. Specifically, this acts on all files and directories marked with f, F, d, D, v, q, Q, p, L, c, b, C,
</span><span style="color:#323232;">w, e. If this switch is used at least one tmpfiles.d/ file (or - for standard input) must be specified on the command line
</span><span style="color:#323232;">or the invocation will be refused, for safety reasons (as otherwise much of the installed system files might be removed).
</span><span style="color:#323232;">
</span><span style="color:#323232;">The primary usecase for this option is to automatically remove files and directories that originally have been created on
</span><span style="color:#323232;">behalf of an installed packaged at package removal time.
</span><span style="color:#323232;">
</span><span style="color:#323232;">It is recommended to first run this command in combination with --dry-run (see below) to verify which files and
</span><span style="color:#323232;">directories will be deleted.
</span><span style="color:#323232;">
</span><span style="color:#323232;">Warning! This is is usually not the command you want! In most cases --remove is what you are looking for.
</span><span style="color:#323232;">
</span><span style="color:#323232;">Added in version 256.
</span>