Document HTTP/3 graceful shutdown and GOAWAY frame behavior

[Copilot]

The documentation lacked detailed explanation of how HTTP/3 graceful shutdown works via GOAWAY frames, covering neither the server-side mechanics nor client-side handling.

Server-side (content/docs/http3/server.md)

• Added "How GOAWAY Works" subsection explaining:
  • GOAWAY frame contents (stream ID indicating highest processed request)
  • Server behavior: stops accepting new connections, rejects new requests with H3_REQUEST_REJECTED, allows in-flight requests to complete
  • Connection lifecycle: QUIC connection remains open for pending requests
• Clarified relationship to HTTP/2's similar GOAWAY mechanism

Client-side (content/docs/http3/client.md)

• Added "Handling Server Shutdown (GOAWAY)" section documenting:
  • Automatic GOAWAY handling by http3.Transport: stops new requests on affected connection, completes in-flight requests, establishes new connections transparently
  • Built-in retry behavior for H3_REQUEST_REJECTED errors with no configuration required

Both sections link to RFC 9114 §5.2 for specification details.

Original prompt

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This section details on the original issue you should resolve

<issue_title>http3: explain graceful shutdown / GOAWAY</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes http3: explain graceful shutdown / GOAWAY #27

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