Document the `query_scope` search index option

[joshdaugherty]

The query_scope option for search indexes, added in statamic/cms#13126 (6.0), isn't documented on the Search page. The page covers filter, and the 5 to 6 upgrade guide already recommends query scopes over filtering for custom searchables, but it doesn't say how to set one on an index.

From Statamic\Search\Searchables\Provider::applyQueryScope() and the built-in searchables on 6.x, these are worth covering:

• The value is a scope handle. The scope is applied to the query before any items are loaded, which is why it's faster than filter on large indexes.
• Unlike filter, it doesn't override the native filters, so draft entries are still left out of the index.
• The entries, terms and users searchables apply it. The assets searchable doesn't.

I'll open a PR for this.