Move explanation of scopes to blocks slide

Which was, confusingly, already named "Blocks and Scopes".

Author: randomPoison

src/control-flow-basics/blocks-and-scopes.md

@@ -4,8 +4,6 @@ minutes: 5
 
 # Blocks and Scopes
 
-## Blocks
-
 A block in Rust contains a sequence of expressions, enclosed by braces `{}`.
 Each block has a value and a type, which are those of the last expression of the
 block:
@@ -19,14 +17,22 @@ fn main() {
         z - y
     };
     println!("x: {x}");
+    // println!("y: {y}");
 }
 ```
 
 If the last expression ends with `;`, then the resulting value and type is `()`.
 
+A variable's scope is limited to the enclosing block.
+
 <details>
 
 - You can show how the value of the block changes by changing the last line in
   the block. For instance, adding/removing a semicolon or using a `return`.
 
+- Demonstrate that attempting to access `y` outside of its scope won't compile.
+
+- Values are effectively "deallocated" when they go out of their scope, even if
+  their data on the stack is still there.
+
 </details>