drivers/cc2420: add datasheet documentation to duplicated strobe call

drivers/cc2420/cc2420.c

@@ -160,6 +160,9 @@ void cc2420_tx_exec(cc2420_t *dev)
 
 static inline void _flush_rx_fifo(cc2420_t *dev)
 {
+    /* as stated in the CC2420 datasheet (section 14.3), the SFLUSHRX command
+     * strobe should be issued twice to ensure that the SFD pin goes back to its
+     * idle state */
     cc2420_strobe(dev, CC2420_STROBE_FLUSHRX);
     cc2420_strobe(dev, CC2420_STROBE_FLUSHRX);
 }