stmhal, usrsw: Update comments for new switch API.

stmhal/usrsw.c

@@ -14,19 +14,15 @@
 
 // Usage Model:
 //
-// pyb.switch() returns True if the user switch is pressed, False otherwise.
+//      sw = pyb.Switch()       # create a switch object
-//
+//      sw()                    # get state (True if pressed, False otherwise)
-// pyb.switch(callback) will register a callback to be called when the user
+//      sw.callback(f)          # register a callback to be called when the
-//                      switch is pressed.
+//                              #   switch is pressed down
-//
+//      sw.callback(None)       # remove the callback
-// pyb.switch(None) will remove the callback.
 //
 // Example:
 //
-// def switch_pressed():
+// pyb.Switch().callback(lambda: pyb.LED(1).toggle())
-//     print("User Switch pressed")
-//
-// pyb.switch(switch_pressed)
 
 // this function inits the switch GPIO so that it can be used
 void switch_init0(void) {