Document New Version Feature Flags

Add some docs around the changes to new version feature flags, including
the strategy-first approach

doc/user/project/operations/feature_flags.md

@@ -91,6 +91,41 @@ NOTE: **NOTE**
 We'd highly recommend you to use the [Environment](../../../ci/environments.md)
 feature in order to quickly assess which flag is enabled per environment.
 
+## Feature flag behavior change in 13.0
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/35555) in GitLab 13.0.
+
+Starting in GitLab 13.0, you can apply a feature flag strategy across multiple environment specs,
+without defining the strategy multiple times.
+
+This feature is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can enable it for your instance.
+
+To enable it:
+
+```ruby
+Feature.enable(:feature_flags_new_version)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:feature_flags_new_version)
+```
+
+### Applying a strategy to environments
+
+After a strategy is defined, it applies to **All Environments** by default. To
+make a strategy apply to a specific environment spec:
+
+1. Click the **Add Environment** button.
+1. Create a new
+   [spec](../../../ci/environments.md#scoping-environments-with-specs).
+
+To apply the strategy to multiple environment specs, repeat these steps.
+
 ## Feature Flag strategies
 
 GitLab Feature Flag adopts [Unleash](https://unleash.github.io)
@@ -158,7 +193,7 @@ To get the access credentials that your application will need to talk to GitLab:
    - **API URL**: URL where the client (application) connects to get a list of feature flags.
    - **Instance ID**: Unique token that authorizes the retrieval of the feature flags.
    - **Application name**: The name of the running environment. For instance,
-       if the application runs for production server, application name would be
+     if the application runs for a production server, application name would be
      `production` or similar. This value is used for
      [the environment spec evaluation](#define-environment-specs).