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.

16_for2.zig

@@ -3,9 +3,15 @@
 // number starting with 0 that counts up with each iteration:
 //
 //     for (items) |item, index| {
+//
 //         // Do something with item and index
+//
 //     }
 //
+// You can name "item" and "index" anything you want. "i" is a popular
+// shortening of "index". The item name is often the singular form of
+// the items you're looping through.
+//
 const std = @import("std");
 
 pub fn main() void {
@@ -15,9 +21,9 @@ pub fn main() void {
     var value: u32 = 0;
 
     // Now we'll convert the binary bits to a number value by adding
-    // the value of the place as a power of two for each bit. See if
-    // you can figure out the missing piece:
+    // the value of the place as a power of two for each bit.
     //
+    // See if you can figure out the missing piece:
     for (bits) |bit, ???| {
         var place_value = std.math.pow(u32, 2, @intCast(u32, i));
         value += place_value * bit;