Documentation: external_url and internal_api_url configuration

If you blindly follow the steps on setting Prefect support for GitLab
it is hard to notice that 'internal_api_url' value for Gitaly
configuration needs to be changed to the proper one. So this commit
fixes it by adding explicit instructions about how it needs to be
changed. Also this commit adds 'external_url' configuration for GitLab
configuration file and comment on how it needs to be setup.

doc/administration/gitaly/praefect.md

@@ -261,7 +261,10 @@ gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'
 # Configure the gitlab-shell API callback URL. Without this, `git push` will
 # fail. This can be your 'front door' GitLab URL or an internal load
 # balancer.
-gitlab_rails['internal_api_url'] = 'https://gitlab.example.com'
+# Possible values could be: 'http://10.23.101.53', 'https://gitlab.example.com',
+# etc. Please replace GITLAB_SERVER_ADDRESS with proper value and change schema
+# to 'https' in case you use encrypted connection.
+gitlab_rails['internal_api_url'] = 'http://GITLAB_SERVER_ADDRESS'
 
 # Replace PRAEFECT_INTERNAL_TOKEN below with a real secret.
 gitaly['auth_token'] = 'PRAEFECT_INTERNAL_TOKEN'
@@ -324,6 +327,12 @@ git_data_dirs({
 
 # Replace GITLAB_SHELL_SECRET_TOKEN below with real secret
 gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'
+
+# Possible values could be: 'http://10.23.101.53', 'https://gitlab.example.com',
+# etc. Please replace GITLAB_SERVER_ADDRESS with proper value and change schema
+# to 'https' in case you use encrypted connection. For more info please refer
+# to https://docs.gitlab.com/omnibus/settings/configuration.html#configuring-the-external-url-for-gitlab
+external_url "http://<GITLAB_SERVER_ADDRESS>"
 ```
 
 Replace `GITLAB_SHELL_SECRET_TOKEN` and `PRAEFECT_EXTERNAL_TOKEN`