Clarify that file:regex over is:watched never works

Change-Id: I5231ae2713954a453d9e311aaf03f5a055c6533f
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index f4328be..3d72732 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -189,7 +189,10 @@
 `file:"^name[1-3].xml"`.
 +
 Currently this operator is only available on a watched project
-and may not be used in the search bar.
+and may not be used in the search bar. The same holds true for web UI
+"My > Watched Changes", i. e. file:regex is used over the is:watched
+expression. It never produces any results, because the error message:
+"operator not permitted here: file:regex" is suppressed.
 
 [[has]]
 has:draft::