document that after_release takes an argument

Looking at https://github.com/moose/Moose/pull/83, I noticed that it's not instantly obvious that an archive must be built because archive_release is passed its filename.

(I suppose you *could* add to the public API that this filename might be undef, but that might cause interesting failures with some existing plugins that aren't expecting it.  Dist::Zilla::Plugin::Run::AfterRelease comes to mind.)