Document redis: Object option

There are four ways to pass the redis option, but only three were
documented. This is now consistent with implementation.

[ci skip]

activesupport/lib/active_support/cache/redis_cache_store.rb

@@ -152,10 +152,12 @@ module ActiveSupport
 
       # Creates a new Redis cache store.
       #
-      # Handles three options: block provided to instantiate, single URL
-      # provided, and multiple URLs provided.
+      # Handles four options: :redis block, :redis instance, single :url
+      # string, and multiple :url strings.
       #
+      #   Option  Class       Result
       #   :redis  Proc    ->  options[:redis].call
+      #   :redis  Object  ->  options[:redis]
       #   :url    String  ->  Redis.new(url: …)
       #   :url    Array   ->  Redis::Distributed.new([{ url: … }, { url: … }, …])
       #