Add <a:documentation> with `docstring` to `p_elem`.

def insert_doc(self, p_elem, docstring):
        """Add <a:documentation> with `docstring` to `p_elem`."""
        dtag = self.namespaces[self.a_uri] + ":documentation"
        elem = SchemaNode(dtag, text=docstring)
        p_elem.annots.append(elem)