Add speaker note that arrays go on the stack (#2826)

src/tuples-and-arrays/arrays.md

@@ -65,4 +65,9 @@ fn main() {
 - Adding `#`, eg `{a:#?}`, invokes a "pretty printing" format, which can be
   easier to read.
 
+- Arrays are not heap-allocated. They are regular values with a fixed size known
+  at compile time, meaning they go on the stack. This can be different from what
+  students expect if they come from a garbage collected language, where arrays
+  may be heap allocated by default.
+
 </details>