Add example of clearing a target question on another question's value change

This will first require updates to:
- [x] JavaRosa (3.1, Feb 2021)
- [x] Collect (1.30, Feb 2021)
- [ ] Validate
- [ ] pyxform

```
Another option for the scenario above is to clear out the value for ``malaria_age`` when ``current_age`` changes. Making ``malaria_age`` a required question will force the enumerator to update ``malaria_age`` if ``current_age`` is corrected.

.. csv-table:: survey
:header: type, name, label, required, calculation, trigger

text, name, Child's name,
integer, current_age, Child's age,
select_one gndr, gender, Gender,
integer, malaria_age, Age at malaria diagnosis, true(), , ${current_age}
```

Start with the issue's proposed CSV-table example and verify whether Validate and pyxform support the trigger behavior, since those updates remain unchecked. Locate the relevant Sphinx documentation page, add the example once both dependencies support it, and confirm the rendered table accurately explains clearing malaria_age when current_age changes.

Written by the indexing model from the issue text.