weston-launch: improve help text for -u and -t

Explain that -u requires root and -t requires -u. Most importantly,
document in what format does -t expect the tty to be given.

It has been confusing, because Weston's --tty option takes an integer,
weston-launch takes a full device path.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

libweston/weston-launch.c

@@ -676,8 +676,10 @@ static void
 help(const char *name)
 {
 	fprintf(stderr, "Usage: %s [args...] [-- [weston args..]]\n", name);
-	fprintf(stderr, "  -u, --user      Start session as specified username\n");
+	fprintf(stderr, "  -u, --user      Start session as specified username,\n"
-	fprintf(stderr, "  -t, --tty       Start session on alternative tty\n");
+			"                  e.g. -u joe, requires root.\n");
+	fprintf(stderr, "  -t, --tty       Start session on alternative tty,\n"
+			"                  e.g. -t /dev/tty4, requires -u option.\n");
 	fprintf(stderr, "  -v, --verbose   Be verbose\n");
 	fprintf(stderr, "  -h, --help      Display this help message\n");
 }