docs/esp8266/general: Start explicit "Known Issues", mentioned RTC inaccuracy.

docs/esp8266/general.rst

@@ -107,8 +107,16 @@ This will allow to keep the structure of your application clear, as well as
 allow to install multiple applications on a board, and switch among them.
 
 
+Known Issues
+------------
+
 Real-time clock
----------------
+~~~~~~~~~~~~~~~
+
+RTC in ESP8266 has very bad accuracy, drift may be seconds per minute. As
+a workaround, to measure short enough intervals you can use
+``utime.time()``, etc. functions, and for wall clock time, synchronize from
+the net using included ``ntpdate.py`` module.
 
 Due to limitations of the ESP8266 chip the internal real-time clock (RTC)
 will overflow every 7:45h.  If a long-term working RTC time is required then