clarify what kind of feature flags we support

doc/development/feature_flags.md

 # Manage feature flags
 
-Starting from GitLab 9.3 we support feature flags via
+Starting from GitLab 9.3 we support feature flags for features in GitLab via
 [Flipper](https://github.com/jnunemaker/flipper/). You should use the `Feature`
 class (defined in `lib/feature.rb`) in your code to get, set and list feature
 flags.
@@ -19,3 +19,8 @@ dynamic (querying the DB etc.).
 
 Once defined in `lib/feature.rb`, you will be able to activate a
 feature for a given feature group via the [`feature_group` param of the features API](../api/features.md#set-or-create-a-feature)
+
+## Feature flags for user applications
+
+GitLab does not yet support the use of feature flags in deployed user applications.
+You can follow the progress on that [in the issue on our issue tracker](https://gitlab.com/gitlab-org/gitlab-ee/issues/779).
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