parse-options: PARSE_OPT_KEEP_UNKNOWN only applies to --options

The description of 'PARSE_OPT_KEEP_UNKNOWN' starts with "Keep unknown
arguments instead of erroring out".  This is a bit misleading, as this
flag only applies to unknown --options, while non-option arguments are
kept even without this flag.

Update the description to clarify this, and rename the flag to
PARSE_OPTIONS_KEEP_UNKNOWN_OPT to make this obvious just by looking at
the flag name.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index e1748fb..bf3c20d 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -1221,7 +1221,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 	revs.sources = &revision_sources;
 	revs.rewrite_parents = 1;
 	argc = parse_options(argc, argv, prefix, options, fast_export_usage,
-			PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN);
+			PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN_OPT);
 	argc = setup_revisions(argc, argv, &revs, NULL);
 	if (argc > 1)
 		usage_with_options (fast_export_usage, options);