Merge branch 'rp/add-to-feature-flags-docs' into 'master'

Add additional info to feature flags development docs

See merge request gitlab-org/gitlab!44154

doc/development/feature_flags/development.md

@@ -440,6 +440,21 @@ Feature.enabled?(:my_feature2) # => false
 Feature.enabled?(:my_feature2, project1) # => true
 ```
 
+### `have_pushed_frontend_feature_flags`
+
+Use `have_pushed_frontend_feature_flags` to test if [`push_frontend_feature_flag`](#frontend)
+has added the feature flag to the HTML.
+
+For example,
+
+```ruby
+stub_feature_flags(value_stream_analytics_path_navigation: false)
+
+visit group_analytics_cycle_analytics_path(group)
+
+expect(page).to have_pushed_frontend_feature_flags(valueStreamAnalyticsPathNavigation: false)
+```
+
 ### `stub_feature_flags` vs `Feature.enable*`
 
 It is preferred to use `stub_feature_flags` to enable feature flags
@@ -496,6 +511,14 @@ Feature.enabled?(:ci_live_trace) # => false
 Feature.enabled?(:ci_live_trace, gate) # => true
 ```
 
+You can also disable a feature flag for a specific actor:
+
+```ruby
+gate = stub_feature_flag_gate('CustomActor')
+
+stub_feature_flags(ci_live_trace: false, thing: gate)
+```
+
 ### Controlling feature flags engine in tests
 
 Our Flipper engine in the test environment works in a memory mode `Flipper::Adapters::Memory`.