docs(spec): describe the API filters as they are built

The /api/schools section still described filtering on stored
school_type_group and faith_groups columns, which the architecture
correction had already dropped: the filters look up the name columns at
request time. Also trims a trailing blank line from two test files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nextjs-app/__tests__/components/FilterBarTypeFaith.test.tsx

@@ -135,4 +135,3 @@ describe('a URL value the options do not spell the same way', () => {
     expect(screen.getByRole('combobox', { name: 'Faith' })).toHaveValue('roman_catholic');
   });
 });
-

nextjs-app/__tests__/components/ResultsToolbar.test.tsx

@@ -176,4 +176,3 @@ describe('the results list filter chips', () => {
     expect(chips).not.toHaveTextContent(/\bcouncil\b(?!-run)/);
   });
 });
-