CPS-464: Document update node leaves limitations

https://lf-onap.atlassian.net/browse/CPS-464

Limitation

Limitation

1

When a fragment (and not a full data tree) is passed as input the validation and creation of the data fail because the structure of data no longer matches the model (if it is compared from root). 

2

You must specify the parent xpath of the node you want to update

3

Extra validation is necessary to prevent key attributes change for mapped list elements

4

The yang parser is not built to expect list value