docs: note benchmark materialize default

- clarify that benchmark timings use materialize="all"

Author: eutwt

docs_api/source/benchmark.rst

@@ -1,8 +1,9 @@
 Benchmark
 =========
 
-These benchmarks show time spent and memory used by `versus.compare()`. The data
-comes from the Python `nycflights13` package (the `weather` table). Each size is
+These benchmarks show time spent and memory used by `versus.compare()` with the
+default `materialize="all"` setting. The data comes from the Python
+`nycflights13` package (the `weather` table). Each size is
 sampled with replacement from the original table and keeps 95% of rows on each
 side. In 4 of the 15 columns (`temp`, `dewp`, `humid`, `wind_dir`), 5% of values
 differ between the two inputs. For the parquet scan case, the sampled tables are