Row select method docs are misleading

[Evertras]

Current docs state:

// Selected sets whether the row is selected or not.

However, this is a little misleading, and caused the following code from #137 to seem reasonable:

for _, r := range m.archTable.SelectedRows() {
	r.Selected( false)
}

The docs should reflect that the row is actually copied and a new version is returned - this is not a mutating action.