Update box-recursive.md (#241)

* Update box-recursive.md

Adding Speaker Notes. This is a high-level guideline what to say, so we can settle on one explanation and review it.
This may be very obvious with the background in some languages, but be very non-obvious in languages that always
use references and never embed structs into structs.

* Fix wording

Co-authored-by: Andrew Walbran <qwandor@google.com>

Author: igor-petruk

src/std/box-recursive.md

@@ -30,3 +30,12 @@ fn main() {
 `- - - - - - - - - - - - -'     '- - - - - - - - - - - - - - - - - - - - - - - -'
 ```
 
+<details>
+    
+If the `Box` was not used here and we attempted to embed a `List` directly into the `List`,
+the compiler would not compute a fixed size of the struct in memory, it would look infinite.
+    
+`Box` solves this problem as it has the same size as a regular pointer and just points at the next
+element of the `List` in the heap.    
+    
+</details>