Restore missing types in reference docs

Certain symbols (e.g. `MakeGoogleDefaultCredentials()`) are missing from our reference docs. The missing types have an `@ingroup foo`. Doxygen renders these as `` tags, which we skip in our docfx -> yaml conversion:

https://github.com/googleapis/google-cloud-cpp/blob/4ce192283831c5f0d815b84462bb53fa294a03af/docfx/doxygen2yaml.cc#L62-L66

The docs for `google-cloud-cpp` version >= `v2.23.0` are affected. When we have a fix, we ought to backport it to the older versions.