add info about redis eviction policy recommended setting

docs/configuration.md

@@ -1484,6 +1484,9 @@ caching information about immutable blobs. Most of the options below control
 how the registry connects to redis. You can control the pool's behavior
 with the [pool](#pool) subsection.
 
+It's advisable to configure Redis itself with the **allkeys-lru** eviction policy
+as the registry does not set an expire value on keys.
+
 <table>
   <tr>
     <th>Parameter</th>