Documentation: update celery monitoring docker usage (#5484)

* docs: update celery monitoring docker usage

* docs: add review comments

Co-Authored-By: shamoon <4887959+shamoon@users.noreply.github.com>

docs/advanced_usage.md

@@ -434,8 +434,10 @@ to view more detailed information about the health of the celery workers
 used for asynchronous tasks. This includes details on currently running,
 queued and completed tasks, timing and more. Flower can also be used
 with Prometheus, as it exports metrics. For details on its capabilities,
-refer to the Flower documentation.
+refer to the [Flower](https://flower.readthedocs.io/en/latest/index.html)
+documentation.
 
+Flower can be enabled with the setting [PAPERLESS_ENABLE_FLOWER](configuration/#PAPERLESS_ENABLE_FLOWER).
 To configure Flower further, create a `flowerconfig.py` and
 place it into the `src/paperless` directory. For a Docker
 installation, you can use volumes to accomplish this:
@@ -444,6 +446,8 @@ installation, you can use volumes to accomplish this:
 services:
   # ...
   webserver:
+    environment:
+      - PAPERLESS_ENABLE_FLOWER
     ports:
       - 5555:5555 # (2)!
     # ...
@@ -452,7 +456,7 @@ services:
 ```
 
 1. Note the `:ro` tag means the file will be mounted as read only.
-2. `flower` runs by default on port 5555, but this can be configured
+2. By default, Flower runs on port 5555, but this can be configured.
 
 ## Custom Container Initialization