Design docs are great for some things. In particular they are good places to document interfaces and contracts for those. However, for interfaces it would be

Design docs are great for some things. In particular they are good places to document interfaces and contracts for those. However, for interfaces it would be better to have architecture documents instead of design docs, with design docs really being internal details. Of course, few companies make this distinction between architecture and design, so in practice design docs are a mix of architecture and design.