hwdb: ieee1394-unit-function: add comment about legacy AV/C devices

hwdb: ieee1394-unit-function: add comment about legacy AV/C devices

It appears that exceptional layout of legacy device requires extra care of
hwdb entry for node device since Linux FireWire subsystem do not pick up
numeric model identifier in vendor directory. In detail, see:

 * https://github.com/systemd/systemd/issues/25029

In the case, udev rule without model attribute is used. Thus hwdb entry
for generic AV/C device should match both cases with and without the
attribute. The wildcard added by a commit 5e577da5f824 ("hwdb: drop model
specifier from general entries") satisfies this condition,

This commit adds comment about it.