firewire: core: add memo about the caller of show functions for device attributes

In the case of firewire core function, the caller of show functions for
device attributes is not only sysfs user, but also device initialization.

This commit adds memo about it against the typical assumption that the
functions are just dedicated to sysfs user.

Link: https://lore.kernel.org/lkml/20240318091759.678326-1-o-takashi@sakamocchi.jp/


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>