Clarify ini file description

As requested in #105

example/gamemode.ini

@@ -43,7 +43,7 @@ inhibit_screensaver=1
 ; Setting this to the keyphrase "accept-responsibility" will allow gamemode to apply GPU optimisations such as overclocks
 ;apply_gpu_optimisations=0
 
-; The device number on the system (usually 0)
+; The DRM device number on the system (usually 0), ie. the number in /sys/class/drm/card0/
 ;gpu_device=0
 
 ; Nvidia specific settings (these are Mhz offsets from the baseline, ie. 0 applies no change)