docs: ActionForm examples for indexing into struct fields (#2017)

Co-authored-by: chrisp60 <gh@cperry.me>

docs/book/src/progressive_enhancement/action_form.md

@@ -56,3 +56,45 @@ let on_submit = move |ev| {
 	}
 }
 ```
+
+## Complex Inputs
+
+Server function arguments that are structs with nested serializable fields should make use of indexing notation of `serde_qs`.
+
+```rust
+use leptos::*;
+use leptos_router::*;
+
+#[derive(serde::Serialize, serde::Deserialize, Debug, Clone)]
+struct HeftyData {
+    first_name: String,
+    last_name: String,
+}
+
+#[component]
+fn ComplexInput() -> impl IntoView {
+    let submit = Action::<VeryImportantFn, _>::server();
+
+    view! {
+      <ActionForm action=submit>
+        <input type="text" name="hefty_arg[first_name]" value="leptos"/>
+        <input
+          type="text"
+          name="hefty_arg[last_name]"
+          value="closures-everywhere"
+        />
+        <input type="submit"/>
+      </ActionForm>
+    }
+}
+
+#[server]
+async fn very_important_fn(
+    hefty_arg: HeftyData,
+) -> Result<(), ServerFnError> {
+    assert_eq!(hefty_arg.first_name.as_str(), "leptos");
+    assert_eq!(hefty_arg.last_name.as_str(), "closures-everywhere");
+    Ok(())
+}
+
+```

router/src/components/form.rs

@@ -365,6 +365,47 @@ fn current_window_origin() -> String {
 /// **Note:** `<ActionForm/>` only works with server functions that use the
 /// default `Url` encoding. This is to ensure that `<ActionForm/>` works correctly
 /// both before and after WASM has loaded.
+///
+/// ## Complex Inputs
+/// Server function arguments that are structs with nested serializable fields
+/// should make use of indexing notation of `serde_qs`.
+///
+/// ```rust
+/// # use leptos::*;
+/// # use leptos_router::*;
+///
+/// #[derive(serde::Serialize, serde::Deserialize, Debug, Clone)]
+/// struct HeftyData {
+///     first_name: String,
+///     last_name: String,
+/// }
+///
+/// #[component]
+/// fn ComplexInput() -> impl IntoView {
+///     let submit = Action::<VeryImportantFn, _>::server();
+///
+///     view! {
+///       <ActionForm action=submit>
+///         <input type="text" name="hefty_arg[first_name]" value="leptos"/>
+///         <input
+///           type="text"
+///           name="hefty_arg[last_name]"
+///           value="closures-everywhere"
+///         />
+///         <input type="submit"/>
+///       </ActionForm>
+///     }
+/// }
+///
+/// #[server]
+/// async fn very_important_fn(
+///     hefty_arg: HeftyData,
+/// ) -> Result<(), ServerFnError> {
+///     assert_eq!(hefty_arg.first_name.as_str(), "leptos");
+///     assert_eq!(hefty_arg.last_name.as_str(), "closures-everywhere");
+///     Ok(())
+/// }
+/// ```
 #[cfg_attr(
     any(debug_assertions, feature = "ssr"),
     tracing::instrument(level = "trace", skip_all,)