Add documentation on some functions

inc/macros.hpp

 #ifndef ECSS_SERVICES_MACROS_HPP
 #define ECSS_SERVICES_MACROS_HPP
 
+/**
+ * Perform an assertion that, if failed, throws an ErrorHandler::Internal error
+ *
+ * @todo Actually hold program execution or throw an exception here
+ */
 #define assertI(cond, error) (ErrorHandler::assertInternal((cond), (error)))
+
+/**
+ * A wrapper for ErrorHandler::assertRequest() that uses `this` as the Message object.
+ *
+ * Only to be used within the Message class.
+ */
 #define assertR(cond, error) (ErrorHandler::assertRequest((cond), *this, (error)))
 
 #endif //ECSS_SERVICES_MACROS_HPP