fexecve(2): call out a scenario where you want !O_EXEC

We note a reason why you might need it, but there's an equally important
reason you may need to omit it: interpreted programs.  Add a note
accordingly, along with the workaround configuration if there's reason
you can't help it.

PR:		294780
Reviewed by:	Jan Bramkamp <crest_freebsd_rlwinm.de>, kib
Differential Revision:	https://reviews.freebsd.org/D56704

lib/libsys/execve.2

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd July 02, 2025
+.Dd April 29, 2026
 .Dt EXECVE 2
 .Os
 .Sh NAME
@@ -232,6 +232,17 @@ is to use the
 .Dv O_EXEC
 flag when opening
 .Fa fd .
+Opening without
+.Dv O_EXEC
+may be necessary in the case of executing an interpreted program, as the
+interpreter will not be able to acquire a descriptor to the script for reading
+without mounting
+.Xr fdescfs 4
+on
+.Pa /dev/fd
+with the
+.Cm nodup
+option.
 Note that the file to be executed can not be open for writing.
 .Sh RETURN VALUES
 As the