vcomp/cmpto_j2k: advertise -O j2k-enc-cpu-conv

The GPU conversion should be better choice in general. The situation
where it can be useful is when the GPU is working hard to manage the
compression alone so that deploying on CPU saves some performance for
the encoder.

src/video_compress/cmpto_j2k.cpp

@@ -466,7 +466,7 @@ static void usage() {
                 }
                 col() << "]";
         }
-        col() << " [--cuda-device <c_index>]\n" << TERM_RESET;
+        col() << "\n\t\t[--cuda-device <c_index>] [--param " CPU_CONV_PARAM "]\n" << TERM_RESET;
 
         col() << "where:\n";
         for(const auto& opt : usage_opts){
@@ -478,6 +478,10 @@ static void usage() {
                 col() << " - " << opt.description << "\n";
         }
         col() << TBOLD("\t<c_index>") << " - CUDA device(s) to use (comma separated)\n";
+        col() << TBOLD("\t--param " CPU_CONV_PARAM)
+              << " - use CPU for pixfmt conversion (useful if GPU\n\t\tis fully "
+                 "occupied by the encoder; an option for decoder exists as "
+                 "well)\n";
 }
 
 #define ASSIGN_CHECK_VAL(var, str, minval) \