docs: cleanup the explanation paragraphs at the start of each exercise.

exercises/smart_pointers/box1.rs

@@ -1,13 +1,15 @@
 // box1.rs
 //
-// At compile time, Rust needs to know how much space a type takes up. This becomes problematic
-// for recursive types, where a value can have as part of itself another value of the same type.
-// To get around the issue, we can use a `Box` - a smart pointer used to store data on the heap,
-// which also allows us to wrap a recursive type.
+// At compile time, Rust needs to know how much space a type takes up. This
+// becomes problematic for recursive types, where a value can have as part of
+// itself another value of the same type. To get around the issue, we can use a
+// `Box` - a smart pointer used to store data on the heap, which also allows us
+// to wrap a recursive type.
 //
-// The recursive type we're implementing in this exercise is the `cons list` - a data structure
-// frequently found in functional programming languages. Each item in a cons list contains two
-// elements: the value of the current item and the next item. The last item is a value called `Nil`.
+// The recursive type we're implementing in this exercise is the `cons list` - a
+// data structure frequently found in functional programming languages. Each
+// item in a cons list contains two elements: the value of the current item and
+// the next item. The last item is a value called `Nil`.
 //
 // Step 1: use a `Box` in the enum definition to make the code compile
 // Step 2: create both empty and non-empty cons lists by replacing `todo!()`