README: changes outside Nix, rollback and uninstall

- Intro: no longer claims that everything reverts by activating an older
  generation; points to the exceptions and steam-frame-nix-cleanup.
- "Changes outside Nix (exceptions)": path, feature, lifetime, removed by
  (the SteamVR debugger key while SteamVR runs, its .armed file, the
  runtime drop-in and restore script, the dashboard patches' saved state,
  hicolor's mtime); what steam-frame-nix-cleanup does and what of older
  versions it removes; what exists only while running (UI patches,
  clipboard-sync, the Firefox desktop user.js, Jellyfin's flatpak run
  options); what install.sh sets up; app data that stays yours.
- Rollback: `cleanup --all` (curl or nix run) after rolling back to a
  generation without steam-frame-nix or older than the cleanup.
- Uninstall: what install.sh uninstall does; dropping steam-frame-nix from
  a kept configuration (cleanup --all first, or `uninstall = true;`).
- Per-module notes and options: icon fallbacks, SteamVR debugger, Firefox,
  Jellyfin; template comment for iconFallbacks.

template/home.nix

@@ -30,8 +30,8 @@
   #     # All programs without Steam's Developer Mode (Konsole, KDE System
   #     # Settings, Dolphin, ... are hidden otherwise):
   #     showAllApps = true;
-  #     # Icon fallbacks for programs without an icon in the menu are on by
-  #     # default; names to provide in addition to what the scan finds:
+  #     # Icon fallbacks (Konsole, KDE System Settings) are on by default;
+  #     # further Breeze icon names (each switch suggests some):
   #     # iconFallbacks.extra = [ "system-file-manager" ];
   #     # Hidden from the menu (listed with Developer Mode or showAllApps):
   #     hiddenApps = [ "lxterminal" "cmake-gui" "firewall-config" "renderdoc" ];