Version 5.1.0 doesn’t exist because, for a short period of time, the Elastic Yum and Apt repositories included unreleased binaries labeled 5.1.0. To avoid confusion and upgrade issues for the people that have installed these without realizing, we decided to skip the 5.1.0 version and release 5.1.1 instead.
You are looking at preliminary documentation for a future release. Not what you want? See the current release documentation.