Rollup merge of #35962 - regexident:compiler-plugin-docs, r=steveklabnik

Updated code sample in chapter on syntax extensions.

The affected API apparently had changed with commit d59accfb065843d12db9180a4f504664e3d23ef1.

---

Further more I had to add

```toml
[lib]
name = "roman_numerals"
crate-type = ["dylib"]
```

to `Cargo.toml` as I otherwise got this compiler error (despite `#![crate_type="dylib"]`):

    [E0457]: plugin `roman_numerals` only found in rlib format, but must be available in dylib format

Might be worth adding a note about that?