The documentation of
package-install says: "Install the package PKG. PKG can be a package-desc [...]", but then it doesn't explain what is the format of a
C-h o package-desc RET should describe it (and actually "package-desc" in the above docstring should be surrounded with `...' so that you can just click on it to get that info. I already fixed it in Emacs's master branch, so don't bother reporting it, tho).