Document that getcwd() and realpath() are built on system calls that

have a different calling convention than the standard function...as seen
in kdump output.
ok deraadt@ schwarze@

src/lib/libc/stdlib/realpath.3

@@ -28,9 +28,9 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: realpath.3,v 1.21 2019/06/15 17:06:46 jmc Exp $
+.\"	$OpenBSD: realpath.3,v 1.22 2019/06/29 21:21:27 guenther Exp $
 .\"
-.Dd $Mdocdate: June 15 2019 $
+.Dd $Mdocdate: June 29 2019 $
 .Dt REALPATH 3
 .Os
 .Sh NAME
@@ -147,3 +147,17 @@ The
 .Fn realpath
 function call first appeared in
 .Bx 4.4 .
+.Pp
+In
+.Ox 6.6 ,
+it was reimplemented on top of a
+.Fn __realpath
+system call whose calling convention differs from the standard
+function by requiring
+.Ar resolved
+to not be
+.Dv NULL
+and by returning an integer,
+zero on success and \-1 with corresponding errno on failure.
+This is visible in the output of
+.Xr kdump 1 .