Document advanced unicorn settings

Currently there are no docs for unicorn for source installations, while for Omnibus we have a document with not much info.

• Migrate http://docs.gitlab.com/omnibus/settings/unicorn.html in CE docs
• Add information from https://gitlab.com/gitlab-org/omnibus-gitlab/blob/b9131bfd9345bd7b8409dc2d4d0c7f61c600b4e7/files/gitlab-config-template/gitlab.rb.template#L385-404