tmpfiles: Add a note that one line is not needed with newer systemd

Signed-off-by: Simon McVittie <smcv@collabora.com>

bus/tmpfiles.d/dbus.conf.in

 # Fields: type; path; mode; uid; gid; age; argument (symlink target)
 
-# Make ${localstatedir}/lib/dbus/
+# Make ${localstatedir}/lib/dbus (required for systemd < 237)
 # Adjust mode and ownership if it already exists.
 d @EXPANDED_LOCALSTATEDIR@/lib/dbus 0755 - - -