_yaml: document node_get()'s default_value arg

#    expected_type (type): The expected type for the value being searched

#    key (str): The key to get a value for in node

#    indices (list of ints): Optionally decend into lists of lists

#    default_value: Optionally return this value if the key is not found

#

# Returns:

#    The value if found in node, otherwise default_value is returned