Consistent instructions and examples

I started off with "hints" that required the poor student to piece
together the information from incomplete bits. A complete example is
like a picture that is worth 1000 words and far clearer.

10_if2.zig

@@ -1,17 +1,22 @@
 //
 // If statements are also valid expressions:
 //
-//   foo = if (a) 2 else 3;
+//     var foo: u8 = if (a) 2 else 3;
 //
-// Note: you'll need to declare a variable type when assigning a value
+// Note: You'll need to declare a variable type when assigning a value
 // from a statement like this because the compiler isn't smart enough
 // to infer the type for you.
 //
+// This WON'T work:
+//
+//     var foo = if (a) 2 else 3; // error!
+//
 const std = @import("std");
 
 pub fn main() void {
     var discount = true;
 
+    // Please use an if...else expression to set "price".
     // If discount is true, the price should be $17, otherwise $20:
     var price = if ???;