docs: Explain why the tests set the number of query threads

Co-authored-by: Andrew Lamb <alamb@influxdata.com>

test_helpers_end_to_end/src/config.rs

@@ -179,6 +179,7 @@ impl TestConfig {
         )
         .with_existing_object_store(ingester_config)
         .with_env("INFLUXDB_IOX_RPC_MODE", "2")
+        // Hard code query threads so query plans do not vary based on environment
         .with_env("INFLUXDB_IOX_NUM_QUERY_THREADS", "4")
     }