Bluetooth: L2CAP: clarify BT_L2CAP_STATUS_OUT

Makes it clearer what that bit means:
If set, the channel has capacity to send at least one PDU.

If unset, the channel ran out of credits and won't be able to send
anything until the peer sends credits back.

Also add debug logs.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>