Update extracted-doctest feature documentation

Author: GuillaumeGomez

src/doc/rustdoc/src/unstable-features.md

@@ -582,6 +582,7 @@ For this rust code:
 ```rust
 /// ```
 /// let x = 12;
+/// Ok(())
 /// ```
 pub trait Trait {}
 ```
@@ -590,28 +591,36 @@ The generated output (formatted) will look like this:
 
 ```json
 {
-  "format_version": 1,
-  "doctests": [
+  "format_version":2,
+  "doctests":[
     {
-      "file": "foo.rs",
-      "line": 1,
-      "doctest_attributes": {
-        "original": "",
-        "should_panic": false,
-        "no_run": false,
-        "ignore": "None",
-        "rust": true,
-        "test_harness": false,
-        "compile_fail": false,
-        "standalone_crate": false,
-        "error_codes": [],
-        "edition": null,
-        "added_css_classes": [],
-        "unknown": []
+      "file":"src/lib.rs",
+      "line":1,
+      "doctest_attributes":{
+        "original":"",
+        "should_panic":false,
+        "no_run":false,
+        "ignore":"None",
+        "rust":true,
+        "test_harness":false,
+        "compile_fail":false,
+        "standalone_crate":false,
+        "error_codes":[],
+        "edition":null,
+        "added_css_classes":[],
+        "unknown":[]
       },
-      "original_code": "let x = 12;",
-      "doctest_code": "#![allow(unused)]\nfn main() {\nlet x = 12;\n}",
-      "name": "foo.rs - Trait (line 1)"
+      "original_code":"let x = 12;\nOk(())",
+      "doctest_code":{
+        "crate_level":"#![allow(unused)]\n",
+        "code":"let x = 12;\nOk(())",
+        "wrapper":{
+          "before":"fn main() { fn _inner() -> core::result::Result<(), impl core::fmt::Debug> {\n",
+          "after":"\n} _inner().unwrap() }",
+          "returns_result":true
+        }
+      },
+      "name":"src/lib.rs - (line 1)"
     }
   ]
 }