system.3: Document bug and workaround when the command name starts with a hypen

man-pages bug: 211029
 https://bugzilla.kernel.org/show_bug.cgi?id=211029

Complete workaround example
(it was too long for the page, but it may be useful here):

......

$ sudo ln -s -T /usr/bin/echo /usr/bin/-echo;
$ cc -o system_hyphen -x c - ;
#include <stdlib.h>

int
main(void)
{
    system(" -echo Hello world!");
    exit(EXIT_SUCCESS);
}

$ ./system_hyphen;
Hello world!

Reported-by: Ciprian Dorin Craciun <ciprian.craciun@gmail.com>
Cc: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>