Rollup merge of #128667 - its-the-shrimp:rustdoc_json_types_rename, r=aDotInTheVoid

rustdoc: normalise type/field names

Updates #100961

- `Import` -> `Use`, to better reflect the terminology of Rust & its syntax
- `TypeBinding` -> `AssocItemConstraint`, to sync up with `clean`
- `FnDecl` -> `FunctionSignature`, because that's what it is
- `Header` -> `FunctionHeader`, because `Header` is a very word that's very heavily loaded with different meanings
- `ItemEnum::AssocType`: `default` -> `type`, because those items appear in `impl` blocks as well, where they're _not_ the "default"
- `ItemEnum::AssocConst`: `default` -> `value`, see the previous point
- `ForeignType` -> `ExternType`, because "foreign" is not the right word there
- boolean fields' names made to consistently be a phrase that can be a yes/no answer, e.g. `async` -> `is_async`

The docs of `ItemEnum::AssocType::type_` & of `ItemEnum::AssocConst::value` are also updated to be up to date with the clarification of the name of the fields

Author: workingjubilee