The code for this text is indeed in the SDK (DLL), in CvGameTextMgr::setBuildingHelp().
This will be called with the parameters bCivilopediaText=false, bTechChooserText=true so you can separate this case from other cases where this is called.
This requires some C++ programming and compiling the DLL.
Unfortunately, since this is done as a widget help text (and not called directly from Python), I don't think you can add text to this string in the Python code (unless you create a new widget, but I'm not sure about that).