chore(api): clarify approximate web search location defaults (#3953)

## Summary

Clarifies how approximate user locations affect web search results.

## Changes

- Document that omitted or null user locations default to the United
States.
- Explain how to avoid the fallback with an approximate location
containing no location fields, or localize results with location fields.

Co-authored-by: marcuswood-oai <323319819+marcuswood-oai@users.noreply.github.com>