xbps-uchroot: document end of options argument

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>

bin/xbps-uchroot/main.c

@@ -86,7 +86,7 @@ static SIMPLEQ_HEAD(bindmnt_head, bindmnt) bindmnt_queue =
 static void __attribute__((noreturn))
 usage(const char *p)
 {
-	printf("Usage: %s [-b src:dest] [-O -t -o <opts>] <dir> <cmd> [<cmdargs>]\n\n"
+	printf("Usage: %s [-b src:dest] [-O -t -o <opts>] [--] <dir> <cmd> [<cmdargs>]\n\n"
 	    "-b src:dest Bind mounts <src> into <dir>/<dest> (may be specified multiple times)\n"
 	    "-O          Creates a tempdir and mounts <dir> read-only via overlayfs\n"
 	    "-t          Creates tempdir and mounts it on tmpfs (for use with -O)\n"

bin/xbps-uchroot/xbps-uchroot.1

@@ -52,6 +52,12 @@ This makes the temporary directory to be mounted in tmpfs, so that everything is
 in RAM. Note that this is only useful if used with the
 .Fl O
 option (overlayfs).
+.It Fl -
+Stop interpreting following arguments as options.
+This option is useful if some of
+.Ar ARGS
+are options passed to
+.Ar COMMAND .
 .El
 .Sh SECURITY
 The