format_code_in_doc_comments = true

Summary:
Another attempt to enable the option.

[We agreed](https://fb.workplace.com/groups/rust.language/posts/24184800024475285) we want it everywhere by default.

# Help us, help yourself

If you can help enabling this option, consider running this command ahead of time:

```
hg files . -I 'your_project/**/*.rs' | \
  xargs arc rustfmt --config format_code_in_doc_comments=true
```

Fix the which is not landed yet:

```
hg st --rev .~1 -n -ma -I '**/*.rs' | \
  xargs arc rustfmt --config format_code_in_doc_comments=true
```

Fix the stack of commits: checkout the top commit, and then:
```
# must include changes from D52632478
fbcode/scripts/nga/stack-rustfmt-new
```

# Previous attempt

D42766542

Reviewed By: zertosh, dtolnay

Differential Revision: D52632085

fbshipit-source-id: f45998c76076470d66339eb99f585856d2114999

rustfmt.toml

@@ -1,6 +1,7 @@
 # Get help on options with `rustfmt --help=config`
 # Please keep these in alphabetical order.
 edition = "2021"
+format_code_in_doc_comments = true
 group_imports = "StdExternalCrate"
 imports_granularity = "Item"
 merge_derives = false