Document -1 as return value.

--HG--
branch : HEAD
diff --git a/man/zip_get_archive_flag.mdoc b/man/zip_get_archive_flag.mdoc
index 9dd708f..b949093 100644
--- a/man/zip_get_archive_flag.mdoc
+++ b/man/zip_get_archive_flag.mdoc
@@ -67,7 +67,8 @@
 .Ar flag
 is set for
 .Ar archive ,
-and 0 if not.
+0 if not,
+and \-1 if an error occurred.
 .Sh SEE ALSO
 .Xr libzip 3 ,
 .Xr zip_set_archive_flag 3