Auto merge of #5346 - derekdreery:package_id_docs, r=matklad
authorbors <bors@rust-lang.org>
Thu, 12 Apr 2018 08:00:38 +0000 (08:00 +0000)
committerbors <bors@rust-lang.org>
Thu, 12 Apr 2018 08:00:38 +0000 (08:00 +0000)
Add docs

Just adds some docs matching what @matklad told me on irc :)


Trivial merge