DDS15 — Relationship between begin/end coherent changes and presentation.coherent_access==true

We observe differences between DDS vendors related to begin/end_coherent_changes and presentation.coherent_access==true. The question is how begin_coherent_changes() should behave when presentation.coherent_access==false. Some vendors are silent and the operation just works, some return a RETCODE_ERROR saying begin_coherent_changes may only be called when presentation.coherent_access==true. To our idea the DDS spec should be precise so that users who separate code and qos (like DDS4CCM) do know what behavior they get