Merge branch 'doc/admin-import-src' into 'master'

[docs] Enable importing from other sites

See merge request gitlab-org/gitlab-ce!23082

doc/user/admin_area/settings/visibility_and_access_controls.md

 # Visibility and access controls
+## Import sources
+
+Choose from which hosting sites the users can
+[import their projects](../../project/import/index.md).
+
+![import sources](img/import_sources.png)
+
 ## Enabled Git access protocols
 > [Introduced][ce-4696] in GitLab 8.10.