warning NULL argument for printf %s would benefit from explanation

[andrew-taylor]

$ cat /tmp/a.c
#include <stdio.h>

int main(void) {
	char *p = NULL;
	printf("%s", p);
}
$ dcc a.c
a.c: In function ‘main’:
a.c:5:17: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
    5 |         printf("%s", p);
      |                 ^~