Add comment to subtle dataclass code (gh-96133)

Add comment to subtle dataclass code (gh-96133)

In the PR that made this change, 1st1 left a "note to self: add a
comment explaining this". This comment was never added.
https://github.com/python/cpython/pull/9518/files#r280608117

I was reading this code and it wasn't obvious to me why we weren't
exec-ing directly into locals. So I got to learn something new :-)
https://docs.python.org/3/reference/executionmodel.html#interaction-with-dynamic-features