Corrected keyword-create examples and added a reference values guide

The search_engines[].location examples throughout these docs showed a location name — "United States" — as the value to send when creating keywords. That value has never been accepted by the API; it always returned 422. The examples have been corrected.

What was wrong

search_engines[].location is not a place name. It's a Google Ads geo-target criteria id, sent as a string, e.g. "2840" for the United States or "1005850" for Dijon, France. Every request example that showed "location": "United States" — in Add a domain and track keywords, Bulk operations, and Usage & quota — has been updated to use the id instead, and the response examples that showed a location object now show its real fields (uuid, code, name, country_iso_code, variant).

What's new

A new Reference values guide lists every accepted value for search_engines[].name, language, and device, and for the top-level frequency — including the full Google language catalogue and every Google country-level location id — and explains how to look up any other location id (Google's own geo-targets list, or reading location.code back from an already-tracked keyword).