enb.conf.example: add comment about device args for 75 PRB cells

srsenb/enb.conf.example

@@ -68,6 +68,7 @@ rx_gain = 40
 
 # For best performance in 2x2 MIMO and >= 15 MHz use the following device_args settings:
 #     USRP B210: num_recv_frames=64,num_send_frames=64
+#     And for 75 PRBs, also append ",master_clock_rate=15.36e6" to the device args
 
 # For best performance when BW<5 MHz (25 PRB), use the following device_args settings:
 #     USRP B210: send_frame_size=512,recv_frame_size=512