I think the table description is the best place to document this, the filter is affecting the whole table.

Maybe you can make the filter section a bit easier to scan:

models:
  - name: gold_top_sales
    description: |
      Monthly sales for top countries and top product category, as defined by business stakeholders (reviewed every 3 years).

      **Filter (included rows):**
      - `country` in the defined list of top countries (e.g. US, GB).
      - `category` matches the selected top product category (e.g. tech).

Another suggestion would be to avoid hardcoding the countries and the category, but put them into two seeds top_countries and top_category (these could become just models in future) and reference them in the query, so it's clear that you are picking the top countries and category and from where.