Document possible edge case with defaults being mistaken as dynamic

- Let us know on the forum what string hit this issue
- Wrap the expression in `string()` to force it to be literal

https://forum.getodk.org/t/url-widget-does-not-allow-sites-that-has-dash-on-the-url/33290/5?u=ln

Start with the linked forum discussion and the issue title to determine which defaults can be mistaken for dynamic expressions. Identify the relevant documentation page from the repository, then document the edge case and the suggested string() workaround, including the forum-requested example string if available.

Written by the indexing model from the issue text.