Document the query_scope search index option

[joshdaugherty]

Closes #1997.

Adds a "Query scopes" section to the Search page, after "Filtering searchables", documenting the query_scope index option from statamic/cms#13126.

It covers what was checked against Statamic\Search\Searchables\Provider and the built-in searchables on 6.x:

• the value is a scope handle, and the scope is applied to the query before items are loaded;
• unlike filter, it doesn't override the native filters, so drafts stay out of the index;
• entries, terms and users apply it, and assets don't.