apps.c: add comment to do_X509_sign() referring to question #19805

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from #21086)

apps/lib/apps.c

@@ -2349,6 +2349,7 @@ int do_X509_sign(X509 *cert, int force_v1, EVP_PKEY *pkey, const char *md,
                              "keyid, issuer", !self_sign))
             goto end;
     }
+    /* May add further measures for ensuring RFC 5280 compliance, see #19805 */
 
     if (mctx != NULL && do_sign_init(mctx, pkey, md, sigopts) > 0)
         rv = (X509_sign_ctx(cert, mctx) > 0);