Add more comments for problem 3 solution

src/p3.c

@@ -35,6 +35,10 @@ double sum_file(const char* filename) {
 	// A more reliable way of repeatedly reading from a file is checking
 	// the return value of fscanf. On success, fscanf will return the number
 	// of values read.
+	//
+	// Do note, using the return value of fscanf means that if an unexpected
+	// value is encountered, we will stop reading and any remaining values
+	// will be skipped.
 	while (fscanf(infile, "%lf", &value) == 1) {
 		printf("(fscanf) Adding value %.2lf to sum\n", value);