Bluetooth: Host: Clarify use of identity in docs

Clarifies the use of identity in `bluetooth.h` by specifying whether the
documentation refers to the identity address, i. e. the actual address
`bt_addr_le_t`, or the identity handle, i. e. an index corresponding to
a specific identity address.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>