loader.conf.5: Add reference to kern.msgbufsize

When booting with boot_verbose, you want a larger msgbuf size. Add a
poitner to its tuneable.

Suggested by:		John De Boskey (Ages ago)
Sponsored by:		Netflix

stand/defaults/loader.conf.5

@@ -21,7 +21,7 @@
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
-.Dd June 12, 2025
+.Dd September 2, 2025
 .Dt LOADER.CONF 5
 .Os
 .Sh NAME
@@ -127,6 +127,9 @@ since those flags apply to all boot loaders.
 .It Ar boot_verbose
 Set to "yes" to get the same effect as boot -v or booting verbose from the
 loader menu.
+See the
+.Va kern.msgbufsize
+tuneable to ensure enough space for the increased number of messages.
 .It Ar exec
 Immediately executes a
 .Xr loader 8