Add example of disabling feature flag for specific actor

doc/development/feature_flags/development.md

@@ -511,6 +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`.