Add DoorbellRegister comment

src/hulk/usb/xhci.d

@@ -35,6 +35,7 @@ struct XHCI
         ushort hci_version;
 
         /**
+         * HCSPARAMS1.
          * 7:0 number of device slots (MaxSlots)
          * 18:8 number of interrupters (MaxIntrs)
          * 23:19 reserved
@@ -320,6 +321,12 @@ struct XHCI
         ulong erdp;
     }
 
+    /**
+     * Located at offset doorbell_offset from PCI base address.
+     *
+     * The number of these is given by HCSPARAMS1.MaxSlots + 1 (for the command
+     * ring).
+     */
     struct DoorbellRegister
     {
         ubyte target;