Document why non-resolvable emails are reject with a generic message

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I204ed759a50ce4db30dd18a36afb13a54d7fb6b4
diff --git a/resources/Documentation/validation.md b/resources/Documentation/validation.md
index 60ee4d2..3f2d639 100644
--- a/resources/Documentation/validation.md
+++ b/resources/Documentation/validation.md
@@ -182,6 +182,13 @@
     * the project from which the file should be imported doesn't permit reads
       (e.g. has the state `HIDDEN`)
 
+**NOTE:** Non-resolvable code owner emails are rejected with a generic message,
+rather than informing about the exact reason why a code owner email is not
+resolvable. This is intentional and prevents that uploaders can probe the
+existence of emails/accounts (if specific reasons would be returned an uploader
+could create a code owner config file with arbitrary emails and then deduce from
+the messages which emails/accounts exist, which would a privacy issue).
+
 **NOTE:** Whether commits that newly add non-resolvable code owners and
 non-resolvable imports are rejected on commit received and on submit is
 controlled by the