Miscellaneous minor improvements (#2370)

src/references/shared.md

@@ -38,6 +38,8 @@ fn x_axis(x: &i32) -> &(i32, i32) {
 
 <details>
 
+- References can never be null in Rust, so null checking is not necessary.
+
 - A reference is said to "borrow" the value it refers to, and this is a good
   model for students not familiar with pointers: code can use the reference to
   access the value, but is still "owned" by the original variable. The course

src/references/slices.md

@@ -20,7 +20,6 @@ fn main() {
 ```
 
 - Slices borrow data from the sliced type.
-- Question: What happens if you modify `a[3]` right before printing `s`?
 
 <details>
 
@@ -43,10 +42,4 @@ fn main() {
 - Slices always borrow from another object. In this example, `a` has to remain
   'alive' (in scope) for at least as long as our slice.
 
-- The question about modifying `a[3]` can spark an interesting discussion, but
-  the answer is that for memory safety reasons you cannot do it through `a` at
-  this point in the execution, but you can read the data from both `a` and `s`
-  safely. It works before you created the slice, and again after the `println`,
-  when the slice is no longer used.
-
 </details>