crypto/x509: document specifically that only v3 certificates are created.

Fixes #21593

Change-Id: I5d6c644ed1d60ae4610712155bae5cf13ee1f886
Reviewed-on: https://go-review.googlesource.com/79876Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/x509.go

@@ -2050,7 +2050,7 @@ func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgori
 // just an empty SEQUENCE.
 var emptyASN1Subject = []byte{0x30, 0}
 
-// CreateCertificate creates a new certificate based on a template.
+// CreateCertificate creates a new X.509v3 certificate based on a template.
 // The following members of template are used: AuthorityKeyId,
 // BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage,
 // IsCA, KeyUsage, MaxPathLen, MaxPathLenZero, NotAfter, NotBefore,