Merge bitcoin/bitcoin#29997: rpc: Remove index-based Arg accessor

fa3169b073 rpc: Remove index-based Arg accessor (MarcoFalke)

Pull request description:

  The index-based Arg accessor is redundant with the name-based one. It does not provide any benefit to the code reader, or otherwise, so remove it.

ACKs for top commit:
  stickies-v:
    re-ACK fa3169b073, addressed doc nits
  achow101:
    ACK fa3169b073
  ryanofsky:
    Code review ACK fa3169b073. One changes since last review are some documentation improvements

Tree-SHA512: f9da1c049dbf38c3b47a8caf8d24d195c2d4b88c7ec45a9ccfb78f1e39f29cb86869f84b308f6e49856b074c06604ab634c90eb89c9c93d2a8169e070aa1bd40

src/rpc/util.cpp

@@ -677,7 +677,7 @@ static const UniValue* DetailMaybeArg(CheckFn* check, const std::vector<RPCArg>&
 
 static void CheckRequiredOrDefault(const RPCArg& param)
 {
-    // Must use `Arg<Type>(i)` to get the argument or its default value.
+    // Must use `Arg<Type>(key)` to get the argument or its default value.
     const bool required{
         std::holds_alternative<RPCArg::Optional>(param.m_fallback) && RPCArg::Optional::NO == std::get<RPCArg::Optional>(param.m_fallback),
     };