docs: add code example to extend_rpc_modules method (#17446)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Jennifer <jenpaff0@gmail.com>

crates/node/builder/src/builder/mod.rs

@@ -547,6 +547,39 @@ where
     }
 
     /// Sets the hook that is run to configure the rpc modules.
+    ///
+    /// This hook can obtain the node's components (txpool, provider, etc.) and can modify the
+    /// modules that the RPC server installs.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,ignore
+    /// use jsonrpsee::{core::RpcResult, proc_macros::rpc};
+    ///
+    /// #[derive(Clone)]
+    /// struct CustomApi<Pool> { pool: Pool }
+    ///
+    /// #[rpc(server, namespace = "custom")]
+    /// impl CustomApi {
+    ///     #[method(name = "hello")]
+    ///     async fn hello(&self) -> RpcResult<String> {
+    ///         Ok("World".to_string())
+    ///     }
+    /// }
+    ///
+    /// let node = NodeBuilder::new(config)
+    ///     .node(EthereumNode::default())
+    ///     .extend_rpc_modules(|ctx| {
+    ///         // Access node components, so they can used by the CustomApi
+    ///         let pool = ctx.pool().clone();
+    ///         
+    ///         // Add custom RPC namespace
+    ///         ctx.modules.merge_configured(CustomApi { pool }.into_rpc())?;
+    ///         
+    ///         Ok(())
+    ///     })
+    ///     .build()?;
+    /// ```
     pub fn extend_rpc_modules<F>(self, hook: F) -> Self
     where
         F: FnOnce(RpcContext<'_, NodeAdapter<T, CB::Components>, AO::EthApi>) -> eyre::Result<()>

crates/node/builder/src/rpc.rs

@@ -283,6 +283,8 @@ where
     }
 
     /// Returns a reference to the configured node.
+    ///
+    /// This gives access to the node's components.
     pub const fn node(&self) -> &Node {
         &self.node
     }

examples/node-custom-rpc/src/main.rs

@@ -32,7 +32,9 @@ fn main() {
     Cli::<EthereumChainSpecParser, RethCliTxpoolExt>::parse()
         .run(|builder, args| async move {
             let handle = builder
+                // configure default ethereum node
                 .node(EthereumNode::default())
+                // extend the rpc modules with our custom `TxpoolExt` endpoints
                 .extend_rpc_modules(move |ctx| {
                     if !args.enable_ext {
                         return Ok(())
@@ -50,6 +52,7 @@ fn main() {
 
                     Ok(())
                 })
+                // launch the node with custom rpc
                 .launch()
                 .await?;