Mention DHCPRELEASE in the "Known RFC Violations" section of the DHCPv4 ARM as a message that doesn't require server-id

There's already the second bullet point that mentions that DHCPDECLINE messages don't have to contain server ID in Kea, despite the RFC saying it MUST. As @wlodek rightfully pointed out in the review of #1119 (closed), Kea also does not enforce server ID existence for DHCPRELEASE messages while the RFC mentions it MUST. So we should also mention DHCPRELEASE in the ARM as an intended violation.