guide: add exhuastive reference docs for #[wasm_bindgen] attributes

guide/src/reference/attributes/on-js-imports/js_class.md

@@ -0,0 +1,20 @@
+# `js_class = "Blah"`
+
+The `js_class` attribute can be used in conjunction with the `method` attribute
+to bind methods of imported JavaScript classes that have been renamed on the
+Rust side.
+
+```rust
+#[wasm_bindgen]
+extern {
+    // We don't want to import JS strings as `String`, since Rust already has a
+    // `String` type in its prelude, so rename it as `JsString`.
+    #[wasm_bindgen(js_name = String)]
+    type JsString;
+
+    // This is a method on the JavaScript "String" class, so specify that with
+    // the `js_class` attribute.
+    #[wasm_bindgen(method, js_class = "String", js_name = charAt)]
+    fn char_at(this: &JsString, index: u32) -> JsString;
+}
+```