supervisor: usb: note that it's unusual to need to call usb_background

Author: jepler

supervisor/usb.h

@@ -29,8 +29,10 @@
 
 #include <stdbool.h>
 
-// Ports must call this as frequently as they can in order to keep the USB connection
-// alive and responsive.
+// Ports must call this as frequently as they can in order to keep the USB
+// connection alive and responsive.  Normally this is called from background
+// tasks after the USB IRQ handler is executed, but in specific circumstances
+// it may be necessary to call it directly.
 void usb_background(void);
 
 // Ports must call this from their particular USB IRQ handler