Auto merge of #5190 - mbrubeck:doc, r=alexcrichton
authorbors <bors@rust-lang.org>
Thu, 15 Mar 2018 22:35:24 +0000 (22:35 +0000)
committerbors <bors@rust-lang.org>
Thu, 15 Mar 2018 22:35:24 +0000 (22:35 +0000)
Remove old note about pre-1.21 versions

None


Trivial merge