added expalantion for const pointer

exercises/040_pointers2.zig

@@ -27,3 +27,19 @@ pub fn main() void {
 
     std.debug.print("a: {}, b: {}\n", .{ a, b.* });
 }
+//
+// A look into the future:
+// When you allocate memory, you store the returned address in
+// a const var. The pointer itself never changes — it always
+// refers to the same allocation — but you can still read and
+// write the data it points to.
+//
+// Example:
+//
+//     const buf = try allocator.alloc(u8, 1024);
+//     buf[0] = 42;  // fine: the *contents* are mutable
+//
+// Note:
+// Passing this pointer to a function is cheap: it's just an address
+// copied on the stack. The caller can work with the data without
+// needing to know where it came from or how it was allocated.