Improve rendering of type-signatures in docs

- [x] Display function arguments using a newline-multiline strategy
    when the signature gets too long. For example:

    ```
    union_with
      ( left left: AssocList<key, value>
      , right right: AssocList<key, value>
      , with with: fn(key, value, value) -> value
      ) -> AssocList<key, value>
    ```

  - [x] Show type-aliases as type-aliases in signatures; provided
    they've been specified as type annotations. Otherwise, fallback to
    the inferred type.

  - [x] Do not show argument names in signatures, but show labels when
    they're present. This reflects more the original intent behind
    labels (which are meant as public-facing documentation).

crates/project/src/docs.rs

@@ -410,6 +410,7 @@ impl DocFunction {
                     .docs_fn_signature(
                         &func_def.name,
                         &func_def.arguments,
+                        &func_def.return_annotation,
                         func_def.return_type.clone(),
                     )
                     .to_pretty_string(MAX_COLUMNS),