net.quic: 13a - correct an overclaimed doc comment

encode_certificate_verify's doc comment stated as settled fact that its
OpenSSL-produced DER signature is directly compatible with net.mbedtls's
verify_ecdsa_signature ("no reformatting needed between the two
libraries"). That's the expected, standard behavior (OpenSSL's default EC
signing format and mbedTLS's ECDSA verification both use ASN.1 DER
ECDSA-Sig-Value, the TLS/X.509 convention), but this repo has never
actually tested it: there's no EC certificate fixture to build an
mbedtls_pk_context from for a real cross-library check, only a same-library
(OpenSSL signs, OpenSSL verifies) round trip. Reworded to state what's
actually verified (source inspection + the same-library test) versus what's
expected-but-untested, rather than asserting settled fact.

Caught during a requested pass verifying claims made across this PR's
commits/comments. Everything else checked (RSA-PSS signing wrapper
non-existence, EC certificate fixture non-existence, the three distinct
key_share wire shapes, the supported_versions client/server asymmetry, and
the application-secrets-derive-after-server-Finished timing) was confirmed
accurate against the cached primary RFC text directly.

Author: Richard Wheeler

vlib/net/quic/tls13_certificate.v

@@ -299,9 +299,17 @@ pub fn encode_certificate_verify(algorithm u16, signing_key ecdsa.PrivateKey, tr
 	// vlib/crypto/ecdsa/ecdsa.v, keyed off the key's own bit size, matching
 	// exactly what sig_scheme_ecdsa_secp256r1_sha256 requires. The
 	// resulting signature is OpenSSL's standard ASN.1 DER ECDSA-Sig-Value
-	// encoding, the same format net.mbedtls's verify_ecdsa_signature (used
-	// on the client-side verify path, tls13_certificate_chain.c.v) already
-	// parses -- no reformatting needed between the two libraries.
+	// encoding (sign_digest, vlib/crypto/ecdsa/ecdsa.v, sets no raw/compact
+	// signature option, so OpenSSL's EVP_PKEY_sign default applies) --
+	// the same format TLS/X.509 ECDSA signatures conventionally use, and
+	// what net.mbedtls's verify_ecdsa_signature (the client-side verify
+	// path, tls13_certificate_chain.c.v) is written to parse. This is
+	// confirmed by source inspection and this file's own same-library
+	// (OpenSSL signs, OpenSSL verifies) round-trip test
+	// (tls13_certificate_test.v), NOT by an actual cross-library
+	// OpenSSL-signs/mbedTLS-verifies test -- this repo has no EC
+	// certificate fixture to build an mbedtls_pk_context from for that
+	// (same gap verify_ecdsa_signature's own client-side tests document).
 	signature := signing_key.sign(content, hash_config: .with_recommended_hash)!
 
 	mut body := []u8{}