Return a better error when a user tries to cast a document that has already been accessed to a value

[lemire]

In #2084, @mmatrosov reported an error with the following code...

      padded_string json(R"({"foo": "bar"})"sv);
      ondemand::parser parser;
      ondemand::document doc = parser.iterate(json);
      // Next line starts iterating through the document as an object, effectively
      // instantiating the object.
      std::string_view foo = doc["foo"];
      // This next line tries to a value instance representing the object as a value.
      // It is not allowed because the object instance is already in play.
      simdjson::ondemand::value value = doc;

Currently, this fails with a somewhat cryptic error "A JSON document made of a scalar (number, Boolean, null or string) is treated as a value" whether we compile in debug or release mode.

Compiling such code in debug mode should now return an OUT_OF_ORDER_ITERATION message which should help debugging.

I have also clarified the documentation.

[lemire]

@mmatrosov Do you approve?

[mmatrosov]

Yeah, sorry, I did not think you was waiting for my approval

[lemire]

@mmatrosov Thanks.

[lemire]

This will be part of the next release.