Bluetooth: Add comment explainging store hint for long term keys

The code itself is not descriptive on what store hint is used for
long term keys and why. So add some extensive comment here. Similar
to what has already been done for identity resolving key store hint.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>