realpath: Belatedly document POSIX conformance

We've been mostly POSIX-conforming since r236400 and fully since r240410,
which fixed a corner case where a missing non-leaf directory would be
reported as ENOTDIR instead of ENOENT.

Sponsored by:	Klara, Inc.
Fixes:		7877ed7ce3 ("Avoid mapping ENOENT to ENOTDIR for non-existent path components.")
Reviewed by:	ziaee, markj
Differential Revision:	https://reviews.freebsd.org/D53027

lib/libc/stdlib/realpath.3

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 11, 2012
+.Dd October 10, 2025
 .Dt REALPATH 3
 .Os
 .Sh NAME
@@ -108,11 +108,11 @@ and
 .Xr getcwd 3 .
 .Sh SEE ALSO
 .Xr getcwd 3
-.\" .Sh STANDARDS
-.\" The
-.\" .Fn realpath
-.\" function conforms to
-.\" .St -p1003.1-2001 .
+.Sh STANDARDS
+The
+.Fn realpath
+function conforms to
+.St -p1003.1-2001 .
 .Sh HISTORY
 The
 .Fn realpath