Update the AdaDelta optimizer class API docs 📗 (#681)
Update the AdaDelta optimizer class API docs 📗 (#681)

- Describe the AdaDelta algorithm in greater detail.
- Add `AdaDelta.init` documentation.

Co-authored-by: Dan Zheng <e8b2cf6f3981c18438c47df51a789b3589ac132d@google.com>
tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis