A reference to a document in a Firestore database.
The document may already exist or can be created by this class.
The components in the document path. This is a series of strings representing each collection and sub-collection ID, as well as the document IDs for any documents that contain a sub-collection (as well as the base document).
The keyword arguments for the constructor. The only supported keyword is client and it must be a Client. It represents the client that created this document reference.
if * the ``path`` is empty * there are an even number of elements * a collection ID in ``path`` is not a string * a document ID in ``path`` is not a string