Merge branch 'docs/rs-implicit-feature-flags' into 'master'

Document implicit feature flag checks

Closes gitlab-org/release/framework#26

See merge request gitlab-org/gitlab-ce!22669

doc/development/rolling_out_changes_using_feature_flags.md

@@ -152,14 +152,31 @@ in RC1, followed by the feature flag being removed in RC2. This in turn means
 the feature will be stable by the time we publish a stable package around the
 22nd of the month.
 
-## Undefined feature flags default to "on"
+## Implicit feature flags
 
-By default, the [`Project#feature_available?`][project-fa],
+The [`Project#feature_available?`][project-fa],
 [`Namespace#feature_available?`][namespace-fa] (EE), and
-[`License.feature_available?`][license-fa] (EE) methods will check if the
-specified feature is behind a feature flag. Unless the feature is explicitly
-disabled or limited to a percentage of users, the feature flag check will
-default to `true`.
+[`License.feature_available?`][license-fa] (EE) methods all implicitly check for
+a feature flag by the same name as the provided argument.
+
+For example if a feature is license-gated, there's no need to add an additional
+explicit feature flag check since the flag will be checked as part of the
+`License.feature_available?` call. Similarly, there's no need to "clean up" a
+feature flag once the feature has reached general availability.
+
+You'd still want to use an explicit `Feature.enabled?` check if your new feature
+isn't gated by a License or Plan.
+
+[project-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/app/models/project_feature.rb#L63-68
+[namespace-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/ee/namespace.rb#L71-85
+[license-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/license.rb#L293-300
+
+### Undefined feature flags default to "on"
+
+An important side-effect of the [implicit feature
+flags][#implicit-feature-flags] mentioned above is that unless the feature is
+explicitly disabled or limited to a percentage of users, the feature flag check
+will default to `true`.
 
 As an example, if you were to ship the backend half of a feature behind a flag,
 you'd want to explicitly disable that flag until the frontend half is also ready
@@ -171,7 +188,3 @@ to be shipped. You can do this via ChatOps:
 
 Note that you can do this at any time, even before the merge request using the
 flag has been merged!
-
-[project-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/app/models/project_feature.rb#L63-68
-[namespace-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/ee/namespace.rb#L71-85
-[license-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/license.rb#L293-300