docs: add citation for Peer Denial of Service

[WesleyRosenblum]

Description of changes:

QUIC§21.9 describes a peer denial of service attack that may be mitigated through tracking connection progress relative to processing:

While there are legitimate uses for all messages, implementations SHOULD track cost of processing relative to progress and treat excessive quantities of any non-productive packets as indicative of an attack. Endpoints MAY respond to this condition with a connection error or by dropping packets.

The existing "Connection Supervisor" introduced in #1097 is suitable for this purpose. I've added a citation for this, and have also mentioned this section in the slow-loris example code.

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