docs: the Background kind describes the fixed compositor

cce-compositor@7a48cfc paints client backgrounds between the grid backdrop
and the cells, so the plate and the README stop describing that as a gap.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

 README.md   |  9 +++++----
 src/main.rs | 13 +++++++------
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index 89fd188..c7a57b4 100644
--- a/README.md
+++ b/README.md
@@ -47,10 +47,11 @@ the selected kind. Tiled is not on the list because a client cannot ask for
 it: the compositor's toggle action tiles a floating window. A `mode_rule` in
 `config.kdl` can force a mode, or circular rendering, for an app_id.
 
-One kind shows a gap in the current compositor rather than a feature: a
-Background-layer child is never visible, because cce draws its desktop grid
-natively above that layer. (A Fullscreen child used to map Floating too, until
-cce-compositor learned to honour client fullscreen requests in-process.)
+Two of these kinds doubled as compositor tests: a Fullscreen child used to
+map Floating, and a Background-layer child used to be buried under the
+natively drawn desktop grid, until cce-compositor learned to honour client
+fullscreen requests in-process and to paint client backgrounds between its
+grid backdrop and the cells.
 
 **XDG** — two buttons, `Open File` and `Save File`, that call the toolkit's
 `file_dialog` module (the XDG Desktop Portal file chooser, via `rfd`) on a
diff --git a/src/main.rs b/src/main.rs
index b8393d8..f70dd61 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -227,12 +227,13 @@ cce-notifier's toasts are\n\
 this kind of surface.",
             ChildKind::LayerBackground => "A wlr-layer-shell surface\n\
 on the Background layer,\n\
-anchored to all four edges,\n\
-behind every window.\n\n\
-cce draws its desktop grid\n\
-natively above this layer,\n\
-so today the surface is\n\
-covered and never seen.",
+anchored to all four edges.\n\n\
+cce paints it over its flat\n\
+backdrop colour and under\n\
+the desktop grid's cells, so\n\
+it shows through the gaps\n\
+between them, like a\n\
+wallpaper would.",
             ChildKind::Status => "An xdg_toplevel whose app_id\n\
 starts with cce-status: the\n\
 WindowRole::StatusBar\n\