Documentation for long options in v4

Ref: https://support.isc.org/Ticket/Display.html?id=21038

A customer has reported some oddities in section 8.2.16 of the ARM:

https://kea.readthedocs.io/en/kea-2.2.0/arm/dhcp4-srv.html#support-for-long-options

• In the "code" line of the example, there appears to be an unnecessary \.

• The comment which seems to say there will be 11 instances of the option 240 in the outgoing packet. Customer states that from the value, option 240 would appear to be around (21 * 12) + 9 bytes long = 261 bytes. Customer suspects there is some confusion with the other option buffer in the test code, which appearsto be 2560 bytes long