Docs: architecture.md names the three particle systems that sample their Birth Rate (#1373)

docs/architecture.md

@@ -368,12 +368,12 @@ Playground cannons without interacting forces evolve every particle independentl
 backend (reached through `EffectHost::particles` when the GPU compositor is active) simulates one
 particle per invocation from the CPU's birth schedule and keeps per-key state checkpoints on the
 GPU, like `SimCache`; the CPU simulation stays the oracle (tests compare ids and positions).
-CC Particle World samples its Birth Rate at every step (`EffectHost::param_at`, #1373), so a
-keyed rate only decides the births of its own time; the rate is left out of the simulation key,
-and a checkpoint is resumed only with the same rate history (the CPU state's `hist`, the GPU
+CC Particle World, CC Particle Systems II and CC Mr. Mercury sample their Birth Rate at every
+step (`EffectHost::param_at`, #1373), so a keyed rate only decides the births of its own time;
+the rate is left out of the simulation key, and a checkpoint is resumed only with the same rate history (the CPU state's `hist`, the GPU
 checkpoint's births). The rate's keyframes are read at the step's exact layer time
 (`EvalCtx::value_at_layer_time`; mapping it to comp time and back rounds under a fractional
-stretch), a rate without keyframes or an expression isn't sampled, and a Particle World layer with
+stretch), a rate without keyframes or an expression isn't sampled, and a layer of one of them with
 expressions isn't layer cached (its key can't see what an expression returned at earlier times).
 `effectcraft-cli bench --gpu` reports CPU vs GPU ms/frame, speed-up, Auto's ms/frame, pixel
 agreement and the steady-state upload / readback MB per frame for every comp plus an