Execution model reference should cover `closure` arg to `exec` & `eval`

[ncoghlan]

When the closure parameter was added to exec and eval for Python 3.11, it rendered the "Interaction with dynamic features" part of the execution model reference slightly out of date: https://docs.python.org/3/reference/executionmodel.html#interaction-with-dynamic-features

The text remains accurate for string execution, but needs updating to correctly cover execution of code objects with closure variable references.

Linked PRs

• gh-122826: Document exec closure name resolution #155014