Guarantee static_method_of uses the right this (#1795)

This came up during #1760 where `Promise.resolve` must be invoked with
`this` as the `Promise` object, but we were erroneously importing it in
such a way that it didn't have a shim and `this` was `undefined`.

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

@@ -23,3 +23,6 @@ let instant = Date::now();
 
 This is similar to the `js_namespace` attribute, but the usage from within Rust
 is different since the method also becomes a static method of the imported type.
+Additionally this attribute also specifies that the `this` parameter when
+invoking the method is expected to be the JS class, e.g. always invoked as
+`Date.now()` instead of `const x = Date.now; x()`.