Add warning text to Custom field Key field description that changing the key value will break integration

We found that changing the key-field string of a custom field will break existing integrations with ADO where the custom field is synced to an ADO field. There is no warning or anything. This is a big risk to us and to the trust that integration requires for our R&D to rely on it with ADO.

My proposal: add a line of text to the existing description of the Custom field Key field warning for the impact a change may have.