Document git-runstatus

I copied most of the text from git-status.txt.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-runstatus.c b/builtin-runstatus.c
index 303c556..0b63037 100644
--- a/builtin-runstatus.c
+++ b/builtin-runstatus.c
@@ -4,7 +4,7 @@
 extern int wt_status_use_color;
 
 static const char runstatus_usage[] =
-"git-runstatus [--color|--nocolor] [--amend] [--verbose]";
+"git-runstatus [--color|--nocolor] [--amend] [--verbose] [--untracked]";
 
 int cmd_runstatus(int argc, const char **argv, const char *prefix)
 {