units: improve Description= string a bit

Let's not use the word "wrapper", as it's not clear what that is, and in
some way any unit file is a "wrapper"... let's simply say that it's
about the runtime directory.

units/user-runtime-dir@.service.in

@@ -8,7 +8,7 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=/run/user/%i mount wrapper
+Description=User runtime directory /run/user/%i
 Documentation=man:user@.service(5)
 After=systemd-user-sessions.service
 StopWhenUnneeded=yes