Comment 3 for bug 1796825

The change to the doc:

**Constraints**

- Before the migrations finish, snapshots cannot be created on the source
  volumes, which could affects the follow-up clone from the same source
  volumes. The typical affected use case is that creating volume-2 via cloning
  volume-1 immediately after creating volume-1 via cloning volume-0. To achieve
  so, users are advised to 1) wait for the first clone finishing, or 2) create
  volume-2 via cloning volume-0 instead of volume-1, or 3) create volume-1 with
  ``--metadata async_migrate=False``.