class FieldPath : NSObject, NSCopying
FieldPath refers to a field in a document. The path may consist of a single field name
(referring to a top level field in the document), or a list of field names (referring to a nested
field in the document).
FieldPathfrom the provided field names. If more than one field name is provided, the path will point to a nested field in a document.
init(_ fieldNames: [String])
A list of field names.
FieldPaththat points to a field location in a document.
A special sentinel
FieldPathto refer to the ID of a document. It can be used in queries to sort or filter by the document ID.
class func documentID() -> Self