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

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

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+# `method`
+
+The `method` attribute allows you to describe methods of imported JavaScript
+objects. It is applied on a function that has `this` as its first parameter,
+which is a shared reference to an imported JavaScript type.
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Set;
+
+    #[wasm_bindgen(method)]
+    fn has(this: &Set, element: &JsValue) -> bool;
+}
+```
+
+This generates a `has` method on `Set` in Rust, which invokes the
+`Set.prototype.has` method in JavaScript.
+
+```rust
+let set: Set = ...;
+let elem: JsValue = ...;
+if set.has(&elem) {
+    ...
+}
+```