Merge branch 'weimeng-update-global-user-permissions-docs' into 'master'

Document global user permission configuration

See merge request gitlab-org/gitlab-ce!25632

doc/administration/index.md

@@ -41,6 +41,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [System hooks](../system_hooks/system_hooks.md): Notifications when users, projects and keys are changed.
 - [Security](../security/README.md): Learn what you can do to further secure your GitLab instance.
 - [Usage statistics, version check, and usage ping](../user/admin_area/settings/usage_statistics.md): Enable or disable information about your instance to be sent to GitLab, Inc.
+- [Global user settings](user_settings.md): Configure instance-wide user permissions.
 - [Polling](polling.md): Configure how often the GitLab UI polls for updates.
 - [GitLab Pages configuration](pages/index.md): Enable and configure GitLab Pages.
 - [GitLab Pages configuration for GitLab source installations](pages/source.md): Enable and configure GitLab Pages on

doc/administration/user_settings.md

@@ -0,0 +1,35 @@
+# Modifying global user settings
+
+GitLab administrators can modify user settings for the entire GitLab instance.
+
+## Disallow users creating top-level groups
+
+By default, new users can create top-level groups. To disable this, modify the appropriate configuration file.
+
+For Omnibus installations, add the following to `/etc/gitlab/gitlab.rb`:
+
+```ruby
+gitlab_rails['gitlab_default_can_create_group'] = false
+```
+
+For source installations, uncomment the following line in `config/gitlab.yml`:
+
+```yaml
+# default_can_create_group: false  # default: true
+```
+
+## Disallow users changing usernames
+
+By default, new users can change their usernames. To disable this, modify the appropriate configuration file.
+
+For Omnibus installations, add the following to `/etc/gitlab/gitlab.rb`:
+
+```ruby
+gitlab_rails['gitlab_username_changing_enabled'] = false
+```
+
+For source installations, uncomment the following line in `config/gitlab.yml`:
+
+```yaml
+# username_changing_enabled: false # default: true - User can change her username/namespace
+```

doc/user/permissions.md

@@ -23,6 +23,12 @@ To add or import a user, you can follow the
 
 See our [product handbook on permissions](https://about.gitlab.com/handbook/product#permissions-in-gitlab)
 
+## Instance-wide user permissions
+
+By default, users can create top-level groups and change their 
+usernames. A GitLab administrator can configure the GitLab instance to
+[modify this behavior](../administration/user_settings.md).
+
 ## Project members permissions
 
 NOTE: **Note:**