Bluetooth: ISO: Document usage of broadcast code

Add a short example of how the broadcast code for BIGs shall
be used.

The spec states:
"On all levels other than UI, the Broadcast Code parameter shall be
 represented as a 128-bit value. The transformation from string to
 number shall be by representing the string in UTF-8, placing the
 resulting bytes in 8-bit fields of the value starting at the least
 significant bit, and then padding with zeros in the most
 significant bits if necessary. For example, the string “Børne
 House” is represented as the value
 0x00000000_6573756F_4820656E_72B8C342.
"

So "Børne House" becomes
[42, C3, B8, 72, 63, 65, 20, 48, 6F, 75, 73, 65, 00, 00, 00, 00]
when sending over HCI.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>