loader.conf: Document boot_verbose

And for the moment, point people to loader.efi(8) for all boot_*
variables.

Sponsored by: Netflix
MFC After: 1 week

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 July 8, 2024
+.Dd February 9, 2025
 .Dt LOADER.CONF 5
 .Os
 .Sh NAME
@@ -116,6 +116,13 @@ option in this manner,
 .Va beastie_disable
 must be set to
 .Dq Li YES .
+.It Ar boot_*
+See list in
+.Xr loader.efi 8
+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.
 .It Ar exec
 Immediately executes a
 .Xr loader 8