docs: name the benchmarks computed from our dataset (review)

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

docs/ARCHITECTURE.md

@@ -61,10 +61,11 @@ history from the full DataFrame and supplementary data from marts. Comparisons
 batch supplementary queries across selected URNs. Async routes still contain
 synchronous dependency calls; a fully asynchronous database layer is not present.
 
-Benchmarks are DfE's own figures, read from marts: `fact_ks2_national_averages`
-and `fact_ks4_national_averages` for England, and `fact_ks4_la_averages` (all
-state-funded schools) for the search rows' "vs LA avg". The API never averages
-school rows to make a benchmark.
+DfE's official benchmarks live in marts: `fact_ks2_national_averages` and
+`fact_ks4_national_averages` for England, and `fact_ks4_la_averages` (all
+state-funded schools, per LA) for the search rows' "vs LA avg".
+`data_loader.compute_benchmarks` computes further state-school benchmarks from
+our own dataset; they are not DfE figures and are labelled as computed.
 
 ## Frontend boundaries