Pg ha updates docs
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Updates PG HA documentation based on feedback from install fest at GitLab Summit
Closes omnibus-gitlab#2901 (closed), omnibus-gitlab#2899 (closed), omnibus-gitlab#2902 (closed), omnibus-gitlab#2903 (closed)
Edited by 🤖 GitLab Bot 🤖