docs(search-api-graphql): state exactly what a fragment costs

[ddeboer]

Follow-up to #916. Its docs said a flattened fragment spread no longer multiplies the cost of what it contains. That holds for a nested fragment, but not for a spread at the operation root: { ...Q } fragment Q on Query { … } still costs its contents 1.5×. A query costing 14.75 inlined costs 25.125 written that way.

• Correct the reference docs and give exact figures from graphql-armor’s cost rule: a nested inline fragment adds 2, a named spread adds 3, a root-level spread costs its contents 1.5×, and a repeat of a named fragment costs 1 plus 1.5× what its first spread scored, even in another operation of the same document.
• Describe the same rules in the JSDoc for maxDepth and maxCost, so they show up in editor hover text.
• Add a test for each case, reading the score from armor’s rejection message, so a graphql-armor upgrade that changes the scoring fails here.
• State that the cost limit bounds a query’s shape, not its result volume: perPage doesn’t enter the cost (armor scales list cost only for first/last), so a page of 100 scores like a page of 1. maxPerPage (default 100) is what bounds result volume. A per-field cost estimator would need graphql-query-complexity in place of armor’s cost plugin; not worth it while maxPerPage holds.