Rollup merge of rust-lang#127290 - its-the-shrimp:document_rustdoc_json_types, r=aDotInTheVoid

Fully document `rustdoc-json-types`

100% of `rustdoc-json-types` is now documented
Here's the summary from rustdoc with `-Zunstable-options --show-coverage`:

```
+-------------------------------------+------------+------------+------------+------------+
| File                                | Documented | Percentage |   Examples | Percentage |
+-------------------------------------+------------+------------+------------+------------+
| src/rustdoc-json-types/lib.rs       |        314 |     100.0% |         23 |      31.9% |
+-------------------------------------+------------+------------+------------+------------+
| Total                               |        314 |     100.0% |         23 |      31.9% |
+-------------------------------------+------------+------------+------------+------------+
```

Author: matthiaskrgr