Say the benchmark's second open no longer pays the backfill

benchmarks.md and catalog_open.rs both said schema::backfill runs on
every Catalog::open. Since ffdd640 it runs on the first open of a path
in a process and is skipped while the stamp matches, so in dr-bench
catalog_open_ms still includes it and catalog_open_warm_ms, the second
open in the same process, no longer does. That is what a library
reopened in one session costs, and both now say which figure is which.
The comment keeps its line count, so no tag below it moves.

tools/bench/src/catalog_open.rs

@@ -14,9 +14,9 @@
 //! without:
 //!
 //! 1. [`Catalog::open`] — connect, migrate if needed, and **backfill**. The
-//!    backfill is the interesting one: `schema::backfill` runs on every open
-//!    and is three passes over the images table, so it is O(library) work on a
-//!    path whose budget is stated in absolute seconds.
+//!    backfill is the interesting one: O(library) passes over the images table
+//!    on a path whose budget is in absolute seconds. Since 0.17.0 it runs on the
+//!    first open in a process and is skipped while nothing changed (`backfilled`).
 //! 2. [`Catalog::count`] — the total, which is what sizes the scrollbar.
 //! 3. [`Catalog::window`] — the first screenful of rows.
 //! 4. [`Catalog::timeline`] — the scrubber's buckets, drawn beside the grid