Merge branch 'jh-add-puma-requirement-to-action-cable-docs' into 'master'

Docs: Puma is the only supported web server for Action Cable

See merge request gitlab-org/omnibus-gitlab!4531

doc/settings/actioncable.md

@@ -7,7 +7,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 # ActionCable
 NOTE: **Note:**
-ActionCable is **experimental** and the features that use this service are behind feature flags.
+ActionCable is **experimental**, and enabling it in the in-app mode also enables experimental features.
 This service is disabled by default. To enable:
@@ -16,12 +16,14 @@ actioncable['enable'] = true
 ```
 By default, ActionCable is run as a separate Puma server that only handles websocket connections. To run ActionCable on
-the existing web server:
+the existing Puma web server:
 ```ruby
 actioncable['in_app'] = true
 ```
+Note: Action Cable is currently not supported for the Unicorn web server in GitLab.
+
 ## Configuring the worker pool size
 ActionCable uses a separate thread pool to handle the websocket connections. The number of threads can be configured