update REFERENCE.md

REFERENCE.md

@@ -2013,6 +2013,7 @@ The following parameters are available in the `redis::instance` defined type:
 * [`acls`](#-redis--instance--acls)
 * [`output_buffer_limit_slave`](#-redis--instance--output_buffer_limit_slave)
 * [`output_buffer_limit_pubsub`](#-redis--instance--output_buffer_limit_pubsub)
+* [`custom_options`](#-redis--instance--custom_options)
 
 ##### <a name="-redis--instance--activerehashing"></a>`activerehashing`
 
@@ -3025,6 +3026,14 @@ Value of client-output-buffer-limit-pubsub in redis config
 
 Default value: `$redis::output_buffer_limit_pubsub`
 
+##### <a name="-redis--instance--custom_options"></a>`custom_options`
+
+Data type: `Hash[String[1],Variant[String[1], Integer]]`
+
+hash of custom options, not available as direct parameter.
+
+Default value: `{}`
+
 ## Functions
 
 ### <a name="redis--get"></a>`redis::get`