feat: VRAM booster greyed out with an NVIDIA card, with why

TECHNICAL.md

@@ -243,7 +243,9 @@ protected, background apps are evicted first. Without it a game that needs
 most of the VRAM (8 GB on the Steam Machine) can be pushed into system RAM
 by the desktop and other apps, and stutter. The kernel side is the `dmem`
 cgroup controller (7.2); amdgpu and Intel's xe register their VRAM with it,
-NVIDIA's driver doesn't. The **VRAM booster** option is only shown when
+NVIDIA's driver doesn't: with an NVIDIA card the option is shown greyed
+out, with why (`WIZARD_VRAM_FAKE_NVIDIA=1` fakes that for tests). Otherwise
+the **VRAM booster** option is only shown when
 `/sys/fs/cgroup/dmem.capacity` lists a VRAM region of at least 2 GB (a
 dedicated GPU; an integrated one registers a small carve-out) or it's already on, and is
 ticked by default; new in 2.3.0, so setups with the SteamOS conversion on