Update documentation of file modification types

PolyGerrit uses 'M' for modified files and 'U' for unchanged files.

Change-Id: Ie46844b8965837a5737952a8a42e2a71191e5511
Signed-off-by: Edwin Kempin <ekempin@google.com>

Documentation/user-review-ui.txt

diff --git a/Documentation/user-review-ui.txt b/Documentation/user-review-ui.txt
index 99ce645..75f2b3f 100644
--- a/Documentation/user-review-ui.txt
+++ b/Documentation/user-review-ui.txt
@@ -307,7 +307,7 @@
 The type of a file modification is indicated by the character in front
 of the file name:
 
-- 'no character' (Modified):
+- `M` or 'no character' (Modified):
 +
 The file existed before this change and is modified.
 
@@ -327,6 +327,10 @@
 +
 The file is new and is copied from an existing file.
 
+- `U` (Unchanged):
++
+The file is unchanged and has the same content.
+
 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"]
 
 [[rename-or-copy]]