Describe the need for `-o` option if multi files given.
MODIFY
mrbgems/mruby-bin-mrbc/tools/mrbc/mrbc.c
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",

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