Per https://go.dev/doc/comment#package:

Every package should have a package comment introducing the package. It provides information relevant to the package as a whole and generally sets expectations for the package. Especially in large packages, it can be helpful for the package comment to give a brief overview of the most important parts of the API, linking to other doc comments as needed.

We should add missing package documentation. See https://pkg.go.dev/github.com/googleapis/librarian: