clarify function of program

pandatool/src/imageprogs/imageTrans.cxx

@@ -27,8 +27,10 @@
 ImageTrans::
 ImageTrans() : ImageFilter(true) {
   set_program_description
-    ("This program reads an image file and writes an essentially equivalent "
-     "image file to the file specified with -o.");
+    ("This program reads an image file and writes a similar "
+     "image file to the output.  It can implicitly convert from one image "
+     "file format to another; it uses the extension of the output filename "
+     "to specify the destination file format.");
 
   add_option
     ("chan", "channels", 50,