diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -1842,6 +1842,9 @@
 def outgoing(repo, subset, x):
     """Changesets not found in the specified destination repository, or the
     default push location.
+
+    If the location resolve to multiple repositories, the union of all
+    outgoing changeset will be used.
     """
     # Avoid cycles.
     from . import (