Again, as Duy pointed out this should be documented.
How about something like this:
diff --git a/Documentation/glossary-content.txt b/Documentation/glossary-content.txt
index f127fe9..781cde3 100644
--- a/Documentation/glossary-content.txt
+++ b/Documentation/glossary-content.txt
@@ -387,7 +387,9 @@ Glob magic is incompatible with literal magic.
exclude;;
After a path matches any non-exclude pathspec, it will be run
through all exclude pathspec (magic signature: `!` or `^`). If it
- matches, the path is ignored.
+ matches, the path is ignored. If only exclude pathspec are given,
+ the exclusion is applied to the result set as if invoked without any
+ pathspec.
--
[[def_parent]]parent::