bluetooth: rename _bt_gatt_ccc and clarify usage

Bluetooth had two public types with similar name _bt_gatt_ccc and
bt_gatt_ccc, but for absolutely different purposes.
That caused misunderstanding of relationship of them and cases
where to use which one.

Commit changes name of _bt_gatt_ccc to more suitable by usage and
improves documentation of it.

Additionally, it changes name of BT_GATT_CCC_INITIALIZER
to correspond the type name.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>