Document when 'U' for unchanged files is used in file list on change screen

Change-Id: I2acb6206e8a8d06663ac80e0916a36efd516fa5b
Signed-off-by: Edwin Kempin <ekempin@google.com>
(cherry picked from commit 89151b05e94bb3ff830b5f668f267feb6a817d87)
diff --git a/Documentation/user-review-ui.txt b/Documentation/user-review-ui.txt
index 75f2b3f..426e6ac 100644
--- a/Documentation/user-review-ui.txt
+++ b/Documentation/user-review-ui.txt
@@ -329,7 +329,9 @@
 
 - `U` (Unchanged):
 +
-The file is unchanged and has the same content.
+The file is unchanged and has the same content. Unchanged files only
+appear in the file list if 2 patch sets are compared and the file has
+comments one of the sides. Otherwise unchanged files are filtered out.
 
 image::images/user-review-ui-change-screen-file-list-modification-type.png[width=800, link="images/user-review-ui-change-screen-file-list-modification-type.png"]