symlink.2: document EOPNOTSUPP

When the file system does not support symbolic links (like in the case
of MSDOS), symlink() returns -1 and sets errno to EOPNOTSUPP.
Document this behavior.

Reviewed by:		glebius, markj
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D49803

lib/libsys/symlink.2

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 30, 2020
+.Dd April 15, 2025
 .Dt SYMLINK 2
 .Os
 .Sh NAME
@@ -92,6 +92,10 @@ or the entire length of either path name exceeded 1023 characters.
 A component of the
 .Fa name2
 path prefix does not exist.
+.It Bq Er EOPNOTSUPP
+The file system containing the file named by
+.Fa name2
+does not support symbolic links.
 .It Bq Er EACCES
 A component of the
 .Fa name2