docs: some comments and TODOs added

example/capacitor_time_curve.cpp

@@ -41,6 +41,8 @@ int main()
 
   for (auto t = 0 * ms; t <= 50 * ms; ++t) {
     const QuantityOf<isq::voltage> auto Vt = V0 * exp(dimensionless(-t / (R * C)));
+    // TODO try to make the below work instead
+    // const QuantityOf<isq::voltage> auto Vt = V0 * exp(-t / (R * C));
 
     std::cout << "at " << t << " voltage is ";