Correct conventional Zig reference vs value passing re #89

exercises/042_pointers4.zig

@@ -1,6 +1,15 @@
 //
 // Now let's use pointers to do something we haven't been
-// able to do before: pass a value by reference to a function!
+// able to do before: pass a value by reference to a function.
+//
+// Why would we wish to pass a pointer to an integer variable
+// rather than the integer value itself? Because then we are
+// allowed to *change* the value of the variable!
+//
+//     +-----------------------------------------------+
+//     | Pass by reference when you want to change the |
+//     | pointed-to value. Otherwise, pass the value.  |
+//     +-----------------------------------------------+
 //
 const std = @import("std");