public relnote: Add comment of quic async signing.

net/quic/core/crypto/quic_crypto_server_config.cc

Index: net/quic/core/crypto/quic_crypto_server_config.cc
diff --git a/net/quic/core/crypto/quic_crypto_server_config.cc b/net/quic/core/crypto/quic_crypto_server_config.cc
index 543a7debd6758e39c7a2f25de4225bdc6755c818..dcc925f3920ca14b2a86b635bc7779a2df15a265 100644
--- a/net/quic/core/crypto/quic_crypto_server_config.cc
+++ b/net/quic/core/crypto/quic_crypto_server_config.cc
@@ -744,6 +744,7 @@ void QuicCryptoServerConfig::ProcessClientHello(
   DCHECK(proof_source_.get());
   string chlo_hash;
   CryptoUtils::HashHandshakeMessage(client_hello, &chlo_hash);
+
   // No need to get a new proof if one was already generated.
   if (!crypto_proof->chain) {
     const QuicTag* tag_ptr;
@@ -1576,6 +1577,12 @@ void QuicCryptoServerConfig::BuildServerConfigUpdateMessage(
           this, version, compressed_certs_cache, common_cert_sets, params,
           std::move(message), std::move(cb)));
 
+  // Note: We unconditionally use the async variant of GetProof here, unlike
+  // elsewhere in this file where we check for the kSYNC tag in the CHLO for the
+  // connection before deciding.  This call is not in the critical serving path,
+  // and so should not have much impact on the experiments associated with that
+  // tag (plus it would be a chore to plumb information about the tag down to
+  // here).
   proof_source_->GetProof(server_ip, params.sni, serialized, version, chlo_hash,
                           connection_options, std::move(proof_source_cb));
 }