Polish expires documentation

src/vmod_tus.vcc

@@ -131,8 +131,11 @@ recommended to be chosen sufficiently small compared to the available
 disk space and disk space be monitored.
 
 The ``expires`` argument (defaulting to 1 day) specifies the duration
-until unfinished uploads expire and are removed. The expiry gets
-extended by that duration with every operation on a file.
+after the last change until uploads are removed.
+
+Finished uploads are also kept for this length of time in order to
+serve the ``Content-Location:`` header. Note that most storage of
+finished uploads can be freed using the `xserver.done()`_ method.
 
 .. XXX warm event? per VCL object?