docs/esp8266/tutorial: Change flash mode from dio to dout.

For some boards, even -fm dio is too fast and they require -fm dout.  This
commit links to the esptool wiki about available flash modes and changes
dio to dout.

docs/esp8266/tutorial/intro.rst

@@ -109,10 +109,12 @@ PC.  You may also need to reduce the baudrate if you get errors when flashing
 that you have.
 
 For some boards with a particular FlashROM configuration (e.g. some variants of
-a NodeMCU board) you may need to use the following command to deploy
-the firmware (note the ``-fm dio`` option)::
+a NodeMCU board) you may need to manually set a compatible
+`SPI Flash Mode <https://github.com/espressif/esptool/wiki/SPI-Flash-Modes>`_.
+You'd usually pick the fastest option that is compatible with your device, but
+the ``-fm dout`` option (the slowest option) should have the best compatibility::
 
-    esptool.py --port /dev/ttyUSB0 --baud 460800 write_flash --flash_size=detect -fm dio 0 esp8266-20170108-v1.8.7.bin
+    esptool.py --port /dev/ttyUSB0 --baud 460800 write_flash --flash_size=detect -fm dout 0 esp8266-20170108-v1.8.7.bin
 
 If the above commands run without error then MicroPython should be installed on
 your board!