Documentation: Include example redis URI with database index (#5653)

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Co-authored-by: shamoon <4887959+shamoon@users.noreply.github.com>

docs/configuration.md

@@ -34,6 +34,8 @@ matcher.
         `redis://<username>:<password>@<host>:<port>`
     -   With the requirepass option PAPERLESS_REDIS =
         `redis://:<password>@<host>:<port>`
+    -   To include the redis database index PAPERLESS_REDIS =
+        `redis://<username>:<password>@<host>:<port>\<DBIndex>`
 
     [More information on securing your Redis
     Instance](https://redis.io/docs/getting-started/#securing-redis).