Confusing docs reference to 3 flags

[hyanwong]

In ancestry.md, we say

"Although this is obviously not a common ancestor event, note that for both of these models each node has to carry at least one of these three flags."

But it is unclear what the 3 flags are to which this sentence refers. Only msprime.NodeType.COMMON_ANCESTOR and msprime.NodeType.PASS_THROUGH are mentioned in that para.