Currently, it seems like
describe-package will only show the contents of the 'commentary' section for packages that are builtin. For example
(describe-package 'python) show a detailed description of the package, however
(describe-package 'persp-mode) only shows the small amount of meta data present for all packages. This despite the fact that it has a "Commentary" section: link
Reading the source of package.el, this seems to be intentional: package.el link, however the GNU docs certainly suggest that the usage of the commentary section as the long description is not specific to built-in packages. Specifically, "If the file has a ‘;;; Commentary:’ section, this section is used as the long description."
Can anyone explain what's going on here?