SDL: note that sdl hook won’t work with split dev packages

pkgs/development/libraries/SDL/setup-hook.sh

@@ -1,6 +1,12 @@
 addSDLPath () {
   if [ -e "$1/include/SDL" ]; then
     export SDL_PATH="${SDL_PATH-}${SDL_PATH:+ }$1/include/SDL"
+    # NB this doesn’t work with split dev packages because different packages
+    # will contain "include/SDL/" and "lib/" directories.
+    #
+    # However the SDL_LIB_PATH is consumed by SDL itself and serves to locate
+    # libraries like SDL2_mixer, SDL2_image, etc which are not split-package
+    # so the check above will only trigger on them.
     if [ -e "$1/lib" ]; then
       export SDL_LIB_PATH="${SDL_LIB_PATH-}${SDL_LIB_PATH:+ }-L$1/lib"
     fi