rustdoc: normalise type/field names

[its-the-shrimp]

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