manual: (fixes #1286) NS_LOG clarification

doc/manual/source/logging.rst

@@ -424,6 +424,11 @@ Guidelines
   Run some example programs with all log components turned on (e.g. 
   ``NS_LOG="***"``).
 
+* Use an explicit cast for any variable of type uint8_t or int8_t, 
+  e.g., ``NS_LOG_LOGIC ("Variable i is " << static_cast<int> (i));``.
+  Without the cast, the integer is interpreted as a char, and the result
+  will be most likely not in line with the expectations.
+  This is a well documented C++ 'feature'.