add documentation

src/platform/qpg6100/Logging.cpp

@@ -125,6 +125,13 @@ extern "C" void otPlatLog(otLogLevel aLogLevel, otLogRegion aLogRegion, const ch
     chip::DeviceLayer::OnLogOutput();
 }
 
+// TODO: have qpg6100 openthread platform implementation defines the APIs.
+// It is not perfect to have the openthread platform calloc/free
+// APIs defined here. If a dedicated source file (e.g. Memory.cpp)
+// that includes only the two functions is used, the target file
+// Memory.o will be thrown away whening linking the libary because
+// there is no one referring the symbols (We are not linking
+// the 'platform' library against openthread).
 extern "C" void * otPlatCAlloc(size_t aNum, size_t aSize)
 {
     return CHIPPlatformMemoryCalloc(aNum, aSize);