Documentation Updates (mcu.delay() and Design Guide) (#481)

/docs/design_guide: added links to firmware build learning guides for SAMD21 & ESP8266. Changes were placed in the "Adding native modules" section, since that seemed to me the best place based on target audience.

Updated documentation for `delay()` which fixes #243.

docs/design_guide.rst

@@ -416,6 +416,13 @@ live in ``shared-module``. If it is port specific then it should live in ``commo
 within the port's folder. In either case, the file and folder structure should
 mimic the structure in ``shared-bindings``.
 
+To test your native modules or core enhancements, follow these Adafruit Learning Guides
+for building local firmware to flash onto your device(s):
+
+`SAMD21 - Build Firmware Learning Guide <https://learn.adafruit.com/micropython-for-samd21/build-firmware>`_
+
+`ESP8266 - Build Firmware Learning Guide <https://learn.adafruit.com/building-and-running-micropython-on-the-esp8266/overview>`_
+
 MicroPython compatibility
 --------------------------------------------------------------------------------
 

shared-bindings/microcontroller/__init__.c

@@ -71,7 +71,11 @@
 //| .. method:: delay_us(delay)
 //|
 //|   Dedicated delay method used for very short delays. **Do not** do long delays
-//|   because it will stall any concurrent code.
+//|   because this stops all other functions from completing. Think of this as an empty
+//|   ``while`` loop that runs for the specified ``(delay)`` time. If you have other
+//|   code or peripherals (e.g audio recording) that require specific timing or
+//|   processing while you are waiting, explore a different avenue such as using 
+//|   `time.sleep()`.
 //|
 STATIC mp_obj_t mcu_delay_us(mp_obj_t delay_obj) {
     uint32_t delay = mp_obj_get_int(delay_obj);