Add comment about split into nominator/denominator

src/time_integration/methods_SSP.jl

@@ -31,6 +31,11 @@ struct SimpleSSPRK33{StageCallbacks} <: SimpleAlgorithmSSP
     stage_callbacks::StageCallbacks
 
     function SimpleSSPRK33(; stage_callbacks = ())
+        # Mathematically speaking, it is not necessary for the algorithm to split the factors
+        # into numerator and denominator. Otherwise, however, rounding errors of the order of
+        # the machine accuracy will occur, which will add up over time and thus endanger the
+        # conservation of the simulation.
+        # See also https://github.com/trixi-framework/Trixi.jl/pull/1640.
         numerator_a = SVector(0.0, 3.0, 1.0) # a = numerator_a / denominator
         numerator_b = SVector(1.0, 1.0, 2.0) # b = numerator_b / denominator
         denominator = SVector(1.0, 4.0, 3.0)