Update README and linker documentation for function wrapping in module instances

crates/wasmtime/README.md

@@ -84,9 +84,10 @@ fn main() -> Result<()> {
     // Create a `Linker` which will be later used to instantiate this module.
     // Host functionality is defined by name within the `Linker`.
     let mut linker = Linker::new(&engine);
-    linker.func_wrap("host", "host_func", |caller: Caller<'_, u32>, param: i32| {
+    linker.root().instance("host").func_wrap("host_func", |ctx: StoreContextMut<'_, ()>, (param,): (i32,)| {
         println!("Got {} from WebAssembly", param);
-        println!("my host state is: {}", caller.data());
+        println!("my host state is: {}", ctx.data());
+        Ok(())
     })?;
 
     // All wasm objects operate within the context of a "store". Each

crates/wasmtime/src/runtime/component/linker.rs

@@ -400,6 +400,12 @@ impl<T: 'static> LinkerInstance<'_, T> {
     /// types that will come from wasm and `Return` is a value coming from the
     /// host going back to wasm.
     ///
+    /// When defining functions that belong to a specific module instance
+    /// (rather than the root linker), you must first create the appropriate instance
+    /// using the [`instance`](LinkerInstance::instance) method and then call `func_wrap`
+    /// on the returned [`LinkerInstance`]. This is necessary to match the nested
+    /// instance structure expected by WebAssembly components.
+    /// 
     /// Additionally the `func` takes a
     /// [`StoreContextMut`](crate::StoreContextMut) as its first parameter.
     ///