Move unicorn settings doc from Omnibus to CE

[ci skip]

doc/administration/unicorn.md

+# Unicorn settings
+
+If you need to adjust the Unicorn timeout or the number of workers you can use
+the following settings in `/etc/gitlab/gitlab.rb`.
+Run `sudo gitlab-ctl reconfigure` for the change to take effect.
+
+```ruby
+unicorn['worker_processes'] = 3
+unicorn['worker_timeout'] = 60
+```
+
+## Advanced settings
+
+Change the following settings only if you really need to.
+
+```
+unicorn['listen'] = '127.0.0.1'
+unicorn['port'] = 8080
+unicorn['socket'] = '/var/opt/gitlab/gitlab-rails/sockets/gitlab.socket'
+unicorn['pidfile'] = '/opt/gitlab/var/unicorn/unicorn.pid'
+unicorn['tcp_nopush'] = true
+unicorn['backlog_socket'] = 1024
+```
+
+Make sure `somaxconn` is equal or higher than `backlog_socket`.
+
+```
+unicorn['somaxconn'] = 1024
+```
+
+We do not recommend changing this setting, but if you really must to:
+
+```
+unicorn['log_directory'] = "/var/log/gitlab/unicorn"
+```
+
+Only change these settings if you understand well what they mean
+see https://about.gitlab.com/2015/06/05/how-gitlab-uses-unicorn-and-unicorn-worker-killer/
+and https://github.com/kzk/unicorn-worker-killer
+
+```
+unicorn['worker_memory_limit_min'] = "300 * 1 << 20"
+unicorn['worker_memory_limit_max'] = "350 * 1 << 20"
+```