add a roff(7) comment marking the API function X509_get_default_private_dir()

as intentionally undocumented because it is trivial and unused in the wild;
OK tb@

Author: schwarze

src/lib/libcrypto/man/X509_LOOKUP_new.3

@@ -1,4 +1,4 @@
-.\" $OpenBSD: X509_LOOKUP_new.3,v 1.3 2021/08/03 19:47:39 schwarze Exp $
+.\" $OpenBSD: X509_LOOKUP_new.3,v 1.4 2021/08/06 19:09:56 schwarze Exp $
 .\"
 .\" Copyright (c) 2021 Ingo Schwarze <schwarze@openbsd.org>
 .\"
@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: August 3 2021 $
+.Dd $Mdocdate: August 6 2021 $
 .Dt X509_LOOKUP_NEW 3
 .Os
 .Sh NAME
@@ -34,6 +34,9 @@
 .Nm X509_get_default_cert_file ,
 .Nm X509_get_default_cert_dir_env ,
 .Nm X509_get_default_cert_file_env
+.\" X509_get_default_private_dir is intentionally undocumented
+.\" because it appears to be unused by any real-world software
+.\" and because it doesn't do much in the first place.
 .Nd certificate lookup object
 .Sh SYNOPSIS
 .In openssl/x509_vfy.h