InstallAttributes: Document rollback dependency

Document that the rollback feature relies on install attributes to be
backwards compatible.

Bug: 1124428
Change-Id: I0a9de16aa5bd749300a05751562fe7c6436c9e38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435192Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Miriam Polzer <mpolzer@google.com>
Cr-Commit-Position: refs/heads/master@{#812616}

components/policy/proto/install_attributes.proto

@@ -8,6 +8,9 @@ option optimize_for = LITE_RUNTIME;
 
 package cryptohome;
 
+// Note: Backwards-incompatible changes of this file might break rollback
+// (go/rollback-data-restore).
+
 message SerializedInstallAttributes {
   message Attribute {
     required string name = 1;