4-arguments URL class constructor is most verbose one and some of other constructors contain
phrase like "This constructor is equivalent with 4-arguments one with following argument:..."

2-arguments constructor contains quite verbose description of URL string parsing and dealing with relative URL. However it specifies nothing about what to do this parsed values of protocol/host/path/port finally.

It would be quite naturally to refer to 4-arguments constructror. However such reference is absent. It means strictly speaking that semantics of parsed URL string is not specified (while from common sense standpoint it looks quite natural to think that behavior of 2 arguments constructor is equivalent with 4 arguments one)