usb: Clarify expected behavior of dev_bin_attrs_are_visible()

The commit "usb: Export BOS descriptor to sysfs" added a binary attribute
group to sysfs. It doesn't check if the descriptors attribute should be
visible, which is by design and not an oversight. Update a comment so that
it better explains this in the dev_bin_attrs_are_visible() function.

Signed-off-by: Elbert Mai <code@elbertmai.com>
Link: https://lore.kernel.org/r/20240306001503.313028-1-code@elbertmai.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>