docs: clarify backfill threshold semantics and RPC config migration

Add doc note that `None` means use backend default for
`set_backfill_thresholds`, and add comment in node-config noting that
RPC config merging moved to the host adapter crate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: prestwich

crates/node-config/src/lib.rs

@@ -14,6 +14,9 @@
 mod core;
 pub use core::{SIGNET_NODE_DEFAULT_HTTP_PORT, SignetNodeConfig};
 
+// NB: RPC config merging (previously `merge_rpc_configs`) is now the
+// responsibility of the host adapter crate (e.g. `signet-host-reth`).
+
 mod storage;
 pub use storage::StorageConfig;
 

crates/node-types/src/notifier.rs

@@ -27,7 +27,8 @@ pub trait HostNotifier {
     /// The backend resolves the block number to a block hash internally.
     fn set_head(&mut self, block_number: u64);
 
-    /// Configure backfill batch size limits.
+    /// Configure backfill batch size limits. `None` means use the backend's
+    /// default.
     fn set_backfill_thresholds(&mut self, max_blocks: Option<u64>);
 
     /// Signal that processing is complete up to this host block number.