Add doc on betty board being the only one that works for VM tests

BUG=b:63011532
TEST=none

Change-Id: I0b4842cf0da1554846baca1766ff667197edd29e
Reviewed-on: https://chromium-review.googlesource.com/552823
Commit-Ready: Jason Clinton <jclinton@chromium.org>
Tested-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: C Shapiro <shapiroc@google.com>
diff --git a/docs/user-doc.md b/docs/user-doc.md
index 509a5c2..6709947 100644
--- a/docs/user-doc.md
+++ b/docs/user-doc.md
@@ -278,6 +278,12 @@
 
         $ ssh -p 9222 root@localhost
 
+-   Warning: After
+    [crbug/710629](https://bugs.chromium.org/p/chromium/issues/detail?id=710629),
+    'betty' is the only board regularly run through pre-CQ and CQ VMTest and so
+    is the most likely to work at ToT. 'betty' is based on 'amd64-generic',
+    though, so 'amd64-generic' is likely to also work for most (non-ARC) tests.
+
 
 ## Result log layout structure