doc: Ability to specify redis database number (#8231)

- Update documentation on how to specify Redis database number

.env.example

@@ -23,6 +23,9 @@ FORCE_SSL=false
 ENABLE_ACCOUNT_SIGNUP=false
 
 # Redis config
+# specify the configs via single URL or individual variables
+# ref: https://www.iana.org/assignments/uri-schemes/prov/redis
+# You can also use the following format for the URL: redis://:password@host:port/db_number
 REDIS_URL=redis://redis:6379
 # If you are using docker-compose, set this variable's value to be any string,
 # which will be the password for the redis service running inside the docker-compose