fix: Correct and enhance the doc on CELERY_BROKER_URL in .env.example (#22693)

Co-authored-by: Jianheng Hou <jianhenh@example.com>

docker/.env.example

 # Celery Configuration
 # ------------------------------
 
-# Use redis as the broker, and redis db 1 for celery broker.
-# Format as follows: `redis://<redis_username>:<redis_password>@<redis_host>:<redis_port>/<redis_database>`
+# Use standalone redis as the broker, and redis db 1 for celery broker. (redis_username is usually set by defualt as empty)
+# Format as follows: `redis://<redis_username>:<redis_password>@<redis_host>:<redis_port>/<redis_database>`.
 # Example: redis://:difyai123456@redis:6379/1
-# If use Redis Sentinel, format as follows: `sentinel://<sentinel_username>:<sentinel_password>@<sentinel_host>:<sentinel_port>/<redis_database>`
-# Example: sentinel://localhost:26379/1;sentinel://localhost:26380/1;sentinel://localhost:26381/1
+# If use Redis Sentinel, format as follows: `sentinel://<redis_username>:<redis_password>@<sentinel_host1>:<sentinel_port>/<redis_database>`
+# For high availability, you can configure multiple Sentinel nodes (if provided) separated by semicolons like below example:
+# Example: sentinel://:difyai123456@localhost:26379/1;sentinel://:difyai12345@localhost:26379/1;sentinel://:difyai12345@localhost:26379/1
 CELERY_BROKER_URL=redis://:difyai123456@redis:6379/1
 CELERY_BACKEND=redis
 BROKER_USE_SSL=false