Clarify JSON-RPC requirements for relay

[bitlogik]

Following this reown-com/blockchain-api#170, eventually the documentation should add everything related to specific needs at the JSON-RPC level.
For example : On top of the JSON-RPC standard, the id value is required to be an integer number, representing the epoch time is millisecond.
For now, the relay documentation only talks about generic JSON-RPC and uses id=1. This id value is not even working with the latest relay running.