Correct the Shift+F2 claim: the keybind does not work in Game Mode

The in-game keys table told users Shift+F2 toggles logging mid-game. That
holds on the desktop, where the in-game Vulkan layer is the logger, but not
in SteamOS Game Mode: there the logger is mangoapp, which ignores the
keybind. Verified under nested gamescope — the keypress produced no log at
all, while mangohudctl in the same harness started one immediately.

Document mangohudctl as the way to start/stop a log by hand in Game Mode.
It drives mangoapp over a SysV message queue, so it needs no display, no
Wayland socket and no window focus.

README.md

@@ -203,9 +203,20 @@ In-game keys (set by the script's MangoHud block):
 
 | Key | Action |
 |---|---|
-| `Shift+F2` | Toggle logging on/off mid-game |
+| `Shift+F2` | Toggle logging on/off mid-game — **desktop only, see below** |
 | Your existing `toggle_hud` key | Hide the HUD overlay (logging keeps running) |
 
+> **`Shift+F2` does not work in SteamOS Game Mode.** There the logger is
+> mangoapp, not the in-game Vulkan layer, and mangoapp ignores the keybind.
+> Verified under nested gamescope: the keypress produced no log at all. To
+> start/stop a log by hand in Game Mode, use `mangohudctl` (ships with MangoHud),
+> which drives mangoapp over a message queue and needs no display or focus:
+>
+> ```bash
+> mangohudctl set log_session true    # start
+> mangohudctl set log_session false   # stop (also writes a _summary.csv)
+> ```
+
 CSV files appear in `~/Downloads/mango-logs/`. Drop them into
 <https://flightlessmango.com/> for graphs, or open in a spreadsheet.