repos: add descriptions from golang.org/pkg/#subrepo
repos: add descriptions from golang.org/pkg/#subrepo

Updates golang/go#36047

Change-Id: I94ce5537cfd37d211f24a911bd983af436c25009
Reviewed-on: https://go-review.googlesource.com/c/build/+/211318
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/build,golang/build,golang/build,golang/build