Revert syntax demo, add note

So as to not overwhelm the reader with syntax that is unseen up to this point, use the original demonstration, but add an explanatory note hinting that there is more to the semantics of string literals than shown.

exercises/006_strings.zig

@@ -7,9 +7,11 @@
 //
 //     const foo = "Hello";
 //
-// Is the same as:
+// Is almost* the same as:
 //
-//     const foo = &[_:0]u8{ 'H', 'e', 'l', 'l', 'o' };
+//     const foo = [_]u8{ 'H', 'e', 'l', 'l', 'o' };
+//
+// (* We'll see what Zig strings REALLY are in Exercise 77.)
 //
 // Notice how individual characters use single quotes ('H') and
 // strings use double quotes ("H"). These are not interchangeable!