Help: Populate tutorial guide text

Migrate tutorial text from individual `directions.txt` files to the main
tutorial document.  Add some comments to source code to provide anchors
for inclusion.

Help/guide/tutorial/Step3/tutorial.cxx

@@ -5,6 +5,7 @@
 
 #include "TutorialConfig.h"
 
+// should we include the MathFunctions header?
 #ifdef USE_MYMATH
 #  include "MathFunctions.h"
 #endif
@@ -12,6 +13,7 @@
 int main(int argc, char* argv[])
 {
   if (argc < 2) {
+    // report version
     std::cout << argv[0] << " Version " << Tutorial_VERSION_MAJOR << "."
               << Tutorial_VERSION_MINOR << std::endl;
     std::cout << "Usage: " << argv[0] << " number" << std::endl;
@@ -20,6 +22,7 @@ int main(int argc, char* argv[])
 
   double inputValue = std::stod(argv[1]);
 
+  // which square root function should we use?
 #ifdef USE_MYMATH
   double outputValue = mysqrt(inputValue);
 #else