man: enhance the description of systemd-stdio-bridge

I hope that this fixes the comment
https://github.com/systemd/systemd/pull/22141#issuecomment-1013960371
> As someone who doesn't know what this prog does

The listing in the man page is sorted according to logical
use: all the options setting the address are now together.

src/stdio-bridge/stdio-bridge.c

@@ -28,7 +28,7 @@ static bool arg_user = false;
 static int help(void) {
 
         printf("%s [OPTIONS...]\n\n"
-               "STDIO or socket-activatable proxy to a given DBus endpoint.\n\n"
+               "Forward messages between two D-Bus busses via a pipe or socket.\n\n"
                "  -h --help              Show this help\n"
                "     --version           Show package version\n"
                "  -p --bus-path=PATH     Path to the bus address (default: %s)\n"