DOC: TerminationValue and note on early termination

Even though ArrivalFunctionToPathFilter is mentioned, the importance of its TerminationValue 
should be stretched even in the  docs of itkSpeedFunctionToPathFilter.
Furthermore, it might help others to note that the path finding can end earlier than expected.

Author: romangrothausmann

include/itkSpeedFunctionToPathFilter.h

@@ -32,7 +32,8 @@ namespace itk
  * For each end- and way-point, a Fast Marching arrival function is
  * computed. The minimal path is extracted for each segment and
  * concatenated to obtain the total path between the start- and
- * end-points, while passing near the given way-points.
+ * end-points, while passing near the given way-points 
+ * (according to the TerminationValue of ArrivalFunctionToPathFilter).
  *
  * The user must provide the following:
  *    1. A real-valued speed function as the filter input
@@ -46,6 +47,8 @@ namespace itk
  * with default settings. Other suitable optimizers include
  * GradientDescentOptimizer and IterateNeighborhoodOptimizer.
  * See itkArrivalFunctionToPathFilter.h for more details.
+ * Note, that the path finding terminates early if the optimizer 
+ * runs out of the image bounds, possibly depending on the speed function.
  *
  * This filter is based on the methods described in:
  * [1] J. Sethian. Level Set Methods and Fast Marching Methods, chapter 20.