Update the AdaGrad optimizer class API docs 📒 (#675)
Update the AdaGrad optimizer class API docs 📒 (#675)

- Describe the AdaGrad algorithm in greater detail.
- Fix documentation for `initialAccumulatorValue` and `epsilon`.
- Add `RMSProp.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