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 {}
 ```
@@ -590,10 +591,10 @@ The generated output (formatted) will look like this:
 
 ```json
 {
-  "format_version": 1,
+  "format_version": 2,
   "doctests": [
     {
-      "file": "foo.rs",
+      "file": "src/lib.rs",
       "line": 1,
       "doctest_attributes": {
         "original": "",
@@ -609,9 +610,17 @@ The generated output (formatted) will look like this:
         "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": "#![allow(dead_code)]\nlet x = 12;\nOk(())",
+      "doctest_code": {
+        "crate_level": "#![allow(unused)]\n#![allow(dead_code)]\n\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)"
     }
   ]
 }
@@ -624,6 +633,10 @@ The generated output (formatted) will look like this:
    * `doctest_attributes` contains computed information about the attributes used on the doctests. For more information about doctest attributes, take a look [here](write-documentation/documentation-tests.html#attributes).
    * `original_code` is the code as written in the source code before rustdoc modifies it.
    * `doctest_code` is the code modified by rustdoc that will be run. If there is a fatal syntax error, this field will not be present.
+     * `crate_level` is the crate level code (like attributes or `extern crate`) that will be added at the top-level of the generated doctest.
+     * `code` is "naked" doctest without anything from `crate_level` and `wrapper` content.
+     * `wrapper` contains extra code that will be added before and after `code`.
+       * `returns_result` is a boolean. If `true`, it means that the doctest returns a `Result` type.
    * `name` is the name generated by rustdoc which represents this doctest.
 
 ### html

src/librustdoc/doctest/tests.rs

@@ -478,6 +478,7 @@ fn get_extracted_doctests(code: &str) -> ExtractedDocTests {
             LangString::default(),
             code.to_string(),
             DUMMY_SP,
+            Vec::new(),
         ),
         &opts,
         Edition::Edition2018,