Merge branch 'docs_select_version_to_install' into 'master'

Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

See merge request !2284

doc/update/8.2-to-8.3.md

 # From 8.2 to 8.3
 
+Make sure you view this update guide from the tag (version) of GitLab you would
+like to install. In most cases this should be the highest numbered production
+tag (without rc in it). You can select the tag in the version dropdown at the
+top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
 **NOTE:** GitLab 8.0 introduced several significant changes related to
 installation and configuration which *are not duplicated here*. Be sure you're
 already running a working version of at least 8.0 before proceeding with this

doc/update/8.3-to-8.4.md

 # From 8.3 to 8.4
 
+Make sure you view this update guide from the tag (version) of GitLab you would
+like to install. In most cases this should be the highest numbered production
+tag (without rc in it). You can select the tag in the version dropdown at the
+top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
 ### 1. Stop server
 
     sudo service gitlab stop

doc/update/8.4-to-8.5.md

 # From 8.4 to 8.5
 
+Make sure you view this update guide from the tag (version) of GitLab you would
+like to install. In most cases this should be the highest numbered production
+tag (without rc in it). You can select the tag in the version dropdown at the
+top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
 ### 1. Stop server
 
     sudo service gitlab stop

doc/update/8.5-to-8.6.md

 # From 8.5 to 8.6
 
+Make sure you view this update guide from the tag (version) of GitLab you would
+like to install. In most cases this should be the highest numbered production
+tag (without rc in it). You can select the tag in the version dropdown at the
+top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
 ### 1. Stop server
 
     sudo service gitlab stop