Improve maturity of RPC endpoint handling and docs in ENSApi

[lightwalker-eth]

Related discussion: #1345 (comment)

1. ENSApi needs to be more proactively clear on exactly how it will operate with RPCs. For example:
   1. What defines the set of chains ENSApi might make RPC calls to?
   2. How should an ENSNode operator decide which RPC endpoints should be provided to ENSApi.
   3. Note: RPC providers may have controls on their side constraining the set of chains an RPC endpoint might use. We need proactive full visibility and predictability on this question.
   4. What happens if there are multiple RPC endpoints provided to ENSApi for the same chain? What will be the resulting behaviour? Does the order of RPC endpoints provided to ENSApi for the same chain have some effect?