Doxygen comments for generated C++ structs

[ottojo]

I noticed that the .msg to .idl conversion preserves comments, and while the template for C structs includes those as doxygen comments (here), the C++ template is still missing those.

This PR intends to add the same comments to the C++ template.

I just copied over the code from the C template, which seems to work. I am not sure however how i can test this in my local workspace with a package containing message definitions and another package containing C++ nodes using those messages... I found the rosidl generate command, but i am not sure how i can use the modified rosidl_generator_cpp when building my messages-package...

[ottojo]

BTW: Is it specified/documented somewhere which/how comments are extracted from .msg to .idl? I noticed, for example, by reading the source code that units are extracted from comments seperately if they follow the pattern (\s*\[([^,\]]+)\]) https://github.com/ros2/rosidl/blob/master/rosidl_adapter/rosidl_adapter/parser.py#L561-L570