discovery: fix docstring of `outgoing` class Also, introduce a more correct name `ancestorsof` for what was named `missingheads` before. For now, we just forward `ancestorsof` to `missingheads` until all users are changed. There were some mistakes in the old docstring / name: * `missingheads` (new name: `ancestorsof`) contains the revs whose ancestors are included in the outgoing operation. It may contain non-head revs and revs which are already on the remote, so the name "missingheads" is wrong in two ways. * `missing` contains only ancestors of `missingheads`, so not *all nodes* present in local but not in remote. * `common` might not contain all common revs, e.g. not some that are not an ancestor of `missingheads`. It seems like the misleading name have fostered an actual bug (issue6372), where `outgoing.missingheads` was used assuming that it contains the heads of the missing changesets.