Add a comment I forgot before

Change-Id: I51ec28d41ac4d87bf15c9ce0a25190f2bf618360
diff --git a/frc971/wpilib/ADIS16448.h b/frc971/wpilib/ADIS16448.h
index 1846723..49c4fb2 100644
--- a/frc971/wpilib/ADIS16448.h
+++ b/frc971/wpilib/ADIS16448.h
@@ -45,6 +45,8 @@
   // readings.
   void operator()();
 
+  // Sets a function to be called immediately after each time this class uses
+  // the SPI bus. This is a good place to do other things on the bus.
   void set_spi_idle_callback(std::function<void()> spi_idle_callback) {
     spi_idle_callback_ = std::move(spi_idle_callback);
   }