Make type of RefreshControl.onRefresh less strict (facebook#22376)

Summary:
I think a common pattern is to use async functions with the onRefresh prop to reload data. I made the type `mixed` since we don't care about what the function returns. Could also be `void | Promise<void>` I haven't really seen a pattern for those in the codebase.
Pull Request resolved: facebook#22376

Differential Revision: D13310372

Pulled By: TheSavior

fbshipit-source-id: 445f28cf2c37f9c937e72d76716002deaf9d9c94

Author: janicduplessis

Libraries/Components/RefreshControl/RefreshControl.js

@@ -87,7 +87,7 @@ export type RefreshControlProps = $ReadOnly<{|
   /**
    * Called when the view starts refreshing.
    */
-  onRefresh?: ?() => void,
+  onRefresh?: ?() => mixed,
 
   /**
    * Whether the view should be indicating an active refresh.