docs: add document on adding class and other attributes to <A/> component (#4086)

Author: gbj

router/src/link.rs

@@ -66,6 +66,25 @@ where
 ///    This is helpful for accessibility and for styling. For example, maybe you want to set the link a
 ///    different color if it’s a link to the page you’re currently on.
 ///
+/// ### Additional Attributes
+///
+/// You can add additional HTML attributes to the `<a>` element created by this component using the attribute
+/// spreading syntax for components. For example, to add a class, you can use `attr:class="my-link"`.
+/// Alternately, you can add any number of HTML attributes (include `class`) after a `{..}` marker.
+///
+/// ```rust
+/// # use leptos::prelude::*; use leptos_router::components::A;
+/// # fn spread_example() -> impl IntoView {
+/// view! {
+///   <A href="/about" attr:class="my-link" {..} id="foo">"Some link"</A>
+///   <A href="/about" {..} class="my-link" id="bar">"Another link"</A>
+///   <A href="/about" {..} class:my-link=true id="baz">"One more"</A>
+/// }
+/// # }
+/// ```
+///
+/// For more information on this attribute spreading syntax, [see here](https://book.leptos.dev/view/03_components.html#spreading-attributes-onto-components).
+///
 /// ### DOM Properties
 ///
 /// `<a>` elements can take several additional DOM properties with special meanings.