Docs: Add valid example that shows vars in a block scope (#14230)

* Docs: Add valid example that shows vars in a block scope

All the current valid examples show hoisting the var to the function scope. This example makes is clear that this rule allows you to declare vars inside a block scope, just not use them out of that scope.

* Docs: Add corresponding invalid example to block-scoped-var

docs/rules/block-scoped-var.md

@@ -34,6 +34,13 @@ function doTryCatch() {
         var f = build;
     }
 }
+
+function doFor() {
+    for (var x = 1; x < 10; x++) {
+        var y = f(x);
+    }
+    console.log(y);
+}
 ```
 
 Examples of **correct** code for this rule:
@@ -71,6 +78,13 @@ function doTryCatch() {
         f = build;
     }
 }
+
+function doFor() {
+    for (var x = 1; x < 10; x++) {
+        var y = f(x);
+        console.log(y);
+    }
+}
 ```
 
 ## Further Reading