This function appends a child to an existing list of children or creates
a new list of children. The child can be created with e.g.
DOMDocument::createElement(),
DOMDocument::createTextNode() etc. or simply by using
any other node.
Raised if this node is of a type that does not allow children of the
type of the node node, or if the node to
append is one of this node's ancestors or this node itself.