Document that ProofSource::GetProof is only used for gQUIC.

Add a comment to ProofSource::GetProof to clarify that it is only used for legacy gQUIC (QUIC_VERSION_46) and not IETF QUIC.

PiperOrigin-RevId: 971320089

quiche/quic/core/crypto/proof_source.h

diff --git a/quiche/quic/core/crypto/proof_source.h b/quiche/quic/core/crypto/proof_source.h
index b9f28bb..c3bb096 100644
--- a/quiche/quic/core/crypto/proof_source.h
+++ b/quiche/quic/core/crypto/proof_source.h
@@ -190,6 +190,9 @@
   // This function may be called concurrently.
   //
   // Callers should expect that |callback| might be invoked synchronously.
+  //
+  // This is used only for "gQUIC" -- that is, version QUIC_VERSION_46, which
+  // does not use TLS, as opposed to "IETF QUIC".
   virtual void GetProof(const QuicSocketAddress& server_address,
                         const QuicSocketAddress& client_address,
                         const std::string& hostname,