loader.efi.8: hw.uart.console is explained in uart

PR:		295944
MFC to:		15
MFC after:	3 days

stand/man/loader.efi.8

@@ -152,9 +152,8 @@ The
 kernel will look at the
 .Dv hw.uart.console
 variable to set its serial console.
-Its format should be described in
-.Xr uart 4
-but is not.
+Its format is described in
+.Xr uart 4 .
 Set it to
 .Dq io:0x3f8,br:115200
 with the proper port address.