wg.4: Document kernel config option

wg(4) can be compiled into the kernel (device wg), but the wg.4 manpage
does not document this. Adjust it to mention this like other drivers do.

MFC after:	3 days
Reviewed by:	carlavilla, kevans, ziaee
Approved by:	carlavilla (mentor)
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1597

share/man/man4/wg.4

@@ -23,14 +23,21 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 12, 2023
+.Dd February 12, 2025
 .Dt WG 4
 .Os
 .Sh NAME
 .Nm wg
 .Nd "WireGuard protocol driver"
 .Sh SYNOPSIS
-To load the driver as a module at boot time, place the following line in
+To compile this driver into the kernel,
+place the following lines in your kernel configuration file:
+.Bd -ragged -offset indent
+.Cd "device wg"
+.Ed
+.Pp
+Alternatively, to load the driver as a module at boot time,
+place the following line in
 .Xr loader.conf 5 :
 .Bd -literal -offset indent
 if_wg_load="YES"