Update the RMSProp optimizer class API docs (#679)
Update the RMSProp optimizer class API docs (#679)

- Describe the RMSProp algorithm in greater detail.
- Add list of RMSProp references.
- Add `RMSProp.init` and `RMSProp.rho` documentation.

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