Clarify comment on NetworkProtocolNumber.
Clarify comment on NetworkProtocolNumber.

The actual values used for this field in Netstack are actually EtherType values
of the protocol in an Ethernet frame. Eg. header.IPv4ProtocolNumber is 0x0800
and not the number of the IPv4 Protocol Number itself which is 4. Similarly
header.IPv6ProtocolNumber is set to 0x86DD whereas the IPv6 protocol number is
41.

See:
  - https://www.iana.org/assignments/ieee-802-numbers/ieee-802-numbers.xhtml (For EtherType)
  - https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml (For ProtocolNumbers)
PiperOrigin-RevId: 328407293

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