FindSDL: Add path suffixes for <prefix>/include/(SDL|SDL12|SDL11)

Also add a comment explaining the purpose of the include/SDL* path
suffixes.  As explained in commit 7cb51739 (FindSDL: Restore
accidentally dropped search paths (#13651

), 2012-11-05) the include/SDL*
path suffixes are for looking in the ENV{SDLDIR} location.
Inspired-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Modules/FindSDL.cmake

@@ -70,7 +70,9 @@
 find_path(SDL_INCLUDE_DIR SDL.h
   HINTS
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL SDL12 SDL11
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
 )
 
 # SDL-1.1 is the name used by FreeBSD ports...