Restore missing types in reference docs

[dbolduc]

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

google-cloud-cpp/docfx/doxygen2yaml.cc

Lines 62 to 66 in 4ce1922

	// Starting with Doxygen 1.9.7 any namespace members that are defined
	// in a group appear as `member` children in the namespace, with just a
	// `refid` pointing to its location. We need to skip these. They lack
	// enough information to create a valid YAML element describing them.
	"member",

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.