sweeper: incorporate @memmett's remarks

see #77 (diff)

Author: torbjoernk

include/pfasst/encap/imex_sweeper.hpp

@@ -234,11 +234,11 @@ namespace pfasst
         }
 
         /**
-         * Spreading initial value to all time nodes.
+         * Compute low-order provisional solution.
+         *
+         * This does not simply copy the initial value to all time nodes but carries out a few
+         * forward/backward IMEX Euler steps between the nodes.
          *
-         * This does not simply copy the initial value to all time nodes but carries out a
-         * Crank-Nicolson step, i.e. a SDC sweep without the integral term.
-         * 
          * @param[in] initial if `true` the explicit and implicit part of the right hand side of the
          *     ODE get evaluated with the initial value
          */