Bluetooth: Host: Doc fixup for rename of BT_SECURITY_L*

BT_SECURITY_LOW and etc. were previously renamed and are no longer
valid.

The original rename is in the following commits:
1c48757d (New names, deprecate old)
5f2a9ba8 (Remove deprecated names)

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemiconductor.no>