#1615: note the frames the renderer measurement keeps in EFFECTCRAFT_CPU_INFLIGHT docs

docs/preferences.md

@@ -93,7 +93,9 @@ Commands (CLI, MCP, control channel):
   frame per thread in flight, so their memory grows with the core count. Set
   `EFFECTCRAFT_CPU_INFLIGHT` to a whole number of frames (at least 1) to cap the frames in
   flight of every render, `effectcraft-cli render` included
-  ([#1615](https://github.com/storytold/effectcraft/issues/1615)).
+  ([#1615](https://github.com/storytold/effectcraft/issues/1615)). One exception: while a job
+  on GPU Acceleration times both compositors on its first frames, it renders them under the
+  cap but keeps them all (up to 12) until it has chosen.
 - `threeD.defaultRenderer`: Default 3D Renderer
 - `labels.N.name` / `labels.N.color`: the 16 label names and colours, used by the Label menu,
   the timeline, project panel, render queue and viewer handles