Add info about have_pushed_frontend_feature_flags to docs

Add example usage of the have_pushed_frontend_feature_flags matcher to
feature flag development docs.

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