Docs: update Geo with upgrade instructions

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Add info on upgrading Geo, for @stanhu - https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6239#note_35442459, https://gitlab.slack.com/archives/C3TRESYPJ/p1500597988745154

Stan pls check if the location makes sense and if it looks good to you? Pls feel free to merge directly if ready.