Document ConstantPathNode fields

[matthew-healy]

This PR updates config.yml to add documentation for the fields of ConstantPathNode.

[matthew-healy]

Not sure why the C documentation generation is failing - I can run doxygen Doxyfile fine locally. Is that check known to be flaky? 🤔

[kddnewton]

Nope, not flaky. Looks like there's a warning about Foo in the output of doxygen in the test run

[matthew-healy]

The issue was that doxygen read the ::Foo in the comment as a reference to some C++ code, which it then couldn't find. Have just removed that part of the comment.

(I couldn't repro it locally initially because I'd forgotten to run bundle exec rake templates 😅).