Clarify NFC QNearFieldTarget::hasNdefMessage function

The hasNdefMessage is a bit unclear and inconsistent either in documentation and/or implementation.

Currently the Android backend returns true for all ndef targets, and the iOS returns true if the tag has an NDEF message, but that requires that the messages are read first. While iOS is more in line with the documentation, it may not be that useful if one is required to read messages first in order to know if there are messages; OTOH Android doesn't strictly behave as documented because an NDEF tag with zero messages returns true as well.

Additional aspect is what is technically possible on each platform / allowed by the underlying APIs.