Document MangoHUD caveats and forcing MSAA/anisotropic filtering on OpenGL applications

Forcing MSAA/anisotropic filtering on OpenGL applications requires the NVIDIA proprietary driver, as Mesa lacks equivalent environment variables (see https://www.reddit.com/r/linux_gaming/comments/93e8c7/forcing_antialiasing_with_mesa_drivers/).

For reference, here's the full command like I'm using for Anachronox via WINE. This forces 4× SSAA + 4× MSAA, 16× anisotropic filtering, 117 FPS limit with in-game V-Sync enabled for low-latency VRR:

__GL_FSAA_MODE=11 __GL_LOG_MAX_ANISO=4 strangle 117 mangohud wine anox