Add documentation for bulk actions in the guide

Release-Notes: skip
Change-Id: I690e66f7117e316fad7a8b8642d28c262266bc19
diff --git a/Documentation/intro-user.txt b/Documentation/intro-user.txt
index 64afd5e..2ae1a50 100644
--- a/Documentation/intro-user.txt
+++ b/Documentation/intro-user.txt
@@ -368,6 +368,10 @@
 project-level]. The project dashboards can be seen in the web UI under
 `Projects` > `List` > <project-name> > `Dashboards`.
 
+All dashboards and search pages allow an action to be applied to multiple
+changes at once. Select the changes with the checkboxes on the left side and
+choose the action from the action bar at the top of the change section.
+
 [[submit]]
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