fs: dlm: update comments about recovery and membership handling

Make clear that a particular recovery iteration must not be aborted
before membership changes are applied to the members list (ls_nodes)
and midcomms layer.  Interrupting recovery before this can result
in missing node-specific changes in midcomms or through lsops.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>