Describe that free-entry fields in field-lists update calculations as each character change is made

Once https://github.com/opendatakit/collect/pull/3023 is merged, https://docs.opendatakit.org/form-question-types/#grouping-multiple-widgets-on-the-same-screen will need to be modified. A section on field list updates and its limitations should also be added.

Start by checking whether ODK Collect PR 3023 has merged and review the referenced “Grouping multiple widgets on the same screen” documentation page. Update that page to describe field-list updates and limitations, including character-by-character calculation updates for free-entry fields; done means the behavior and limitations are documented accurately.

Written by the indexing model from the issue text.