Add docs and remove typecheck from variadic attribute

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

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+# Variadic Parameters
+
+In javascript, both the types of function arguments, and the number of function arguments are
+dynamic. For example
+
+```js
+function sum(...rest) {
+    let i;
+    // the old way
+    let old_way = 0;
+    for (i=0; i<arguments.length; i++) {
+        old_way += arguments[i];
+    }
+    // the new way
+    let new_way = 0;
+    for (i=0; i<rest.length; i++) {
+        new_way += rest[i];
+    }
+    // both give the same answer
+    assert(old_way === new_way);
+    return new_way;
+}
+```
+
+This function doesn't translate directly into rust, since we don't currently support variadic
+arguments on the wasm target. To bind to it, we use a slice as the last argument, and annotate the
+function as variadic:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(variadic)]
+    fn sum(args: &[i32]) -> i32;
+}
+```
+
+when we call this function, the last argument will be expanded as the javascript expects.
+