If a "docstring" feature is present it will still override a Doxygen comment.
If the "autodoc" feature is also present, the combined "autodoc" and "docstring"
will override the Doxygen comment. If no "docstring" is present then the
"autodoc" feature will not be generated when there is a Doxygen comment.
This way the "autodoc" feature can be specified and used to provide documentation
for 'missing' Doxygen comments.
Closes#1635