doc(random_wallpaper): Explain that we use swaybg for Wayland

bin/random_wallpaper

@@ -2,7 +2,8 @@
 
 # random_wallpaper
 #
-# Requirement: feh (apt install feh)
+# Requirement: feh (apt install feh) - for Xorg
+# Requirement: swaybg - for Wayland
 
 directory="$HOME/wallpapers"
 minutes=15
@@ -50,6 +51,7 @@ while true; do
   # Change background
   file="${array_files["$NUMBER"]}"
   if "${is_wayland}"; then
+    # Use of swaybg program (used with swaymsg)
     $PROGRAM output "*" bg "$directory/$file" fill
   else
     feh --bg-fill "$directory/$file"