Describe the need for `-o` option if multi files given.

mrbgems/mruby-bin-mrbc/tools/mrbc/mrbc.c

@@ -33,7 +33,7 @@ usage(const char *name)
   static const char *const usage_msg[] = {
   "switches:",
   "-c           check syntax only",
-  "-o<outfile>  place the output into <outfile>",
+  "-o<outfile>  place the output into <outfile>; required for multi-files",
   "-v           print version number, then turn on verbose mode",
   "-g           produce debugging information",
   "-B<symbol>   binary <symbol> output in C language format",