Better document supervise-daemon's `retry` option

[N-R-K]

retry is criminally underdocumented. I also see there's some "forever" stuff in there that's not mentioned anywhere in the docs.

Originally posted by @N-R-K in #958

[N-R-K]

Parsing code for reference:

openrc/src/shared/schedules.c

Lines 144 to 169 in e638d4b

	static SCHEDULEITEM *parse_schedule_item(const char *applet, const char *string)
	{
	const char *after_hyph;
	int sig;
	SCHEDULEITEM *item = xmalloc(sizeof(*item));
	item->value = 0;
	item->gotoitem = NULL;
	if (strcmp(string,"forever") == 0)
	item->type = SC_FOREVER;
	else if (isdigit((unsigned char)string[0])) {
	item->type = SC_TIMEOUT;
	errno = 0;
	if (sscanf(string, "%d", &item->value) != 1)
	eerrorx("%s: invalid timeout value in schedule '%s'",
	applet, string);
	} else if ((after_hyph = string + (string[0] == '-')) &&
	((sig = parse_signal(applet, after_hyph)) != -1))
	{
	item->type = SC_SIGNAL;
	item->value = (int)sig;
	} else
	eerrorx("%s: invalid schedule item `%s'", applet, string);
	return item;
	}