Add validate_address RPC method to RPC documentation
This commit adds an addition RPC method: https://github.com/monero-project/monero/pull/5141/commits/5c81a9f1a113e6932b019ba9f0d72b9ccd7cf743
The RPC method should be documented within the RPC documentation: https://www.getmonero.org/resources/developer-guides/wallet-rpc.html