[Docs] MULTI_GPU: say when a layer split pays on a slow CPU

docs/MULTI_GPU.md

@@ -423,6 +423,14 @@ The Coder on an RTX 5080 + RTX 3090 (Ryzen 9 9950X3D), 32K context; details in
 - Leave out a much slower card when two already hold the model. An RTX 2080 Ti as a third card made the 5080 +
   3090 pair slower (68 / 90 tok/s decode): every extra card costs its own round per window.
 - More cards pay off when the model's routed experts do not fit the faster ones.
+- **The split pays most when the CPU pool is the bottleneck.** On two identical RTX 2080 Ti
+  22 GB over a dual-socket Xeon E5-2696 v4 (AVX2 only, 88 logical CPUs), a plain split moved
+  decode from 35.2 to 51.0 tok/s (+45%) and an 11.6K-token prompt from 672 to 908 tok/s (+35%)
+  at 180 W, engine 0.1.49, six 300-token coding prompts. `STRATA_DECODE_TIMING=1` shows why:
+  the window fell from 70.06 to 37.96 ms, almost entirely because the CPU pool fell from 9.52
+  to 0.79 ms per window. The "on par" number above is a fast CPU (9950X3D); when the CPU pool
+  is what the window waits on, moving half the layers to a second identical card removes that
+  work and pays about 1.45x.
 
 ## One conversation with both cards busy (`--pipeline-windows`, default on two or more AMD gfx12 stages)