Explain why creating context variables in closures can extend their lifetimes: a reachable Context holds strong references to variables with values set in it. Repeated creation and binding in a long-lived context can accumulate variables.

Keep the recommendation to create variables at module level while describing the actual retention condition instead of saying they cannot be properly garbage collected.

Fixes #131837.

Validation:

• Used a weak-referenceable default value to confirm retention by a reachable context, collection after discarding the context, and collection after resetting the variable to its unset state.
• Full HTML documentation build with warnings treated as errors.
• Sphinx lint and patchcheck.

• Issue: Improve ContextVar in closures documentation #131837