[doc] Homogenize the format of the c++ docstrings and add more doxygen tags in some places

[tmadlener]

BEGINRELEASENOTES

• Make all docstrings of c++ classes use the /// style (instead of the previous mix of /// and /**/`.
• Extend and improve existing docstrings (e.g. add @params and @returns tags, add information or reword).

ENDRELEASENOTES