docs/esp8266/general: Add more points to "Multitude of boards" section.

docs/esp8266/general.rst

@@ -10,4 +10,25 @@ There are multitude of modules and boards from different sources which carry
 ESP8266 chip. MicroPython tries to provide a generic port which would run on
 as many boards/modules as possible, but there may be limitations. Adafruit
 Feather HUZZAH board is taken as a reference board for the port (for example,
-testing is performed on it).
+testing is performed on it). If you have another board, please make sure you
+have datasheet, schematics and other reference materials for your board
+handy to look up various aspects of your board functioning.
+
+To make a generic ESP8266 port and support as many boards as possible,
+following design and implementation decision were made:
+
+* GPIO pin numbering is based on ESP8266 chip numbering, not some "logical"
+numbering of a particular board. Please have manual/pin diagram of your board
+handy to find correspondce between your board pins and actual ESP8266 pins.
+We also encourage users of various boards to share this mapping via MicroPython
+forum, with the idea to collect community-maintained reference materials
+eventually.
+
+* All pins which make sense to support, are supported by MicroPython
+(for example, we don't expose pins which are used to connect SPI flash
+are not exposed, as they're unlikely useful for anything else, and
+operating on them will lead to board lock-up). However, any particular
+board may expose only subset of pins. Consult your board reference manual.
+
+* Some boards may lack external pins/internal connectivity to support
+ESP8266 deepsleep mode.