Clarify how to set the Assignee field when submitting a branch.
As Ulrike pointed out: - The previous phrasing was very unclear. - It's hard to guess whether the FT is able and willing to handle a given review. Better let them decide. - It's not obvious how one can try to find a reviewer by themselves; while regular contributors should hopefully have no trouble doing so, new contributors would be left in doubt. So let's make the default case "empty the Assignee field" and whoever is handling the FT's frontdesk role at a given time (for now: yours truly; some day: rotating role) will dispatch reviews adequately. And hopefully, other team leads have some kind of view that helps the detect pending reviews in their own area, without relying on the FT. refs: #16650