Replace placement calls with region nodes

tests/chain_cost.rs

@@ -1,6 +1,5 @@
-//! What the vertex shader's move-chain walk costs, against how deep the chain
-//! is. Every active widget owns a slot, so the depth a primitive resolves
-//! through is its depth in the widget tree.
+//! What the vertex shader's move-chain walk costs, against how many nested
+//! region nodes a primitive resolves through.
 //!
 //!     cargo test --release --test chain_cost -- --ignored --nocapture
 //!