docs: Add doc to FromNative._is_native

Related (#2393 (comment))

Author: dangotbanned

narwhals/_translate.py

@@ -218,4 +218,6 @@ class FromNative(Protocol[FromNativeT]):
     @classmethod
     def from_native(cls, data: FromNativeT, *args: Any, **kwds: Any) -> Self: ...
     @staticmethod
-    def _is_native(obj: FromNativeT | Any, /) -> TypeIs[FromNativeT]: ...
+    def _is_native(obj: FromNativeT | Any, /) -> TypeIs[FromNativeT]:
+        """Return `True` if `obj` can be passed to `from_native`."""
+        ...