docs: standardize code documentation style

[rdmark]

The inline code documentation is still quite inconsistent. The majority open up with /*! but others /** which I believe are two different doxygen dialects. The breakdown is 140 vs 34, so I think we should standardize on the former.

Also, there are straggler incorrect or inconsistent tag usage, for instance only intermittent use of @brief for the short summary at the top.

Edit: As per the doxygen docs, /*! is called the Qt style and /** the javadoc style.

Edit2: The Qt project itself refers to the same as QDoc comments, to be parsed by a tool called qdoc.

[rdmark]

done as a part of #2485

the code comment standard is documented in https://github.com/Netatalk/netatalk/blob/main/CONTRIBUTING.md#rules now