Tweaks from student feedback (#1061)

* help students with the luhn exercise

* mention method-specific types in speaker notes

* Update src/exercises/day-2/luhn.md

Co-authored-by: Martin Geisler <martin@geisler.net>

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

src/exercises/day-2/luhn.md

@@ -6,8 +6,8 @@ following to validate the credit card number:
 
 * Ignore all spaces. Reject number with less than two digits.
 
-* Moving from right to left, double every second digit: for the number `1234`,
-  we double `3` and `1`.
+* Moving from **right to left**, double every second digit: for the number `1234`,
+  we double `3` and `1`. For the number `98765`, we double `6` and `8`.
 
 * After doubling a digit, sum the digits. So doubling `7` becomes `14` which
   becomes `5`.
@@ -16,8 +16,10 @@ following to validate the credit card number:
 
 * The credit card number is valid if the sum ends with `0`.
 
-Copy the following code to <https://play.rust-lang.org/> and implement the
-function:
+Copy the code below to <https://play.rust-lang.org/> and implement the function.
+
+Try to solve the problem the "simple" way first, using `for` loops and integers.
+Then, revisit the solution and try to implement it with iterators.
 
 
 ```rust

src/std/hashmap.md

@@ -61,6 +61,8 @@ fn main() {
   but it can lead into complications with the borrow checker.
   * Try removing `to_string()` from the example above and see if it still compiles. Where do you think we might run into issues?
 
+* This type has several "method-specific" return types, such as `std::collections::hash_map::Keys`. These types often appear in searches of the Rust docs. Show students the docs for this type, and the helpful link back to the `keys` method.
+
 [1]: https://doc.rust-lang.org/std/collections/hash_map/struct.HashMap.html#impl-From%3C%5B(K,+V);+N%5D%3E-for-HashMap%3CK,+V,+RandomState%3E
 
 </details>