Update docstrings for aget/aset to be consistent with Clojure
The docstrings can be updated to match Clojure in these respects:
Reflect variadic nature.
For aset, indicate the return value.
Additionally, the parameter i can be renamed idx to match Clojure.
Attached a patch which updates the docstrings and also the param names. Since param names were changed, added unit tests (including covering the runtime and macro forms).