Make it more clear how shared_extra_conf is combined in our Docker configuration scripts (#19323)

For reference, this PR used to include this whole `shared_config` block  in the diff.

But https://github.com/element-hq/synapse/pull/19324 was merged first which introduced parts of it already.

Here is what this code used to look like: 566670c363/docker/configure_workers_and_start.py (L865-L868)

---

Original context for why it was changed this way:
https://github.com/matrix-org/synapse/pull/14921#discussion_r1126257933


Previously, this code made me question two things:

1. Do we actually use `worker_config["shared_extra_conf"]` in the
templates?
- At first glance, I couldn't see why we're updating `shared_extra_conf`
here. It's not used in the `worker.yaml.j2` template so all of this
seemed a bit pointless.
- Turns out, updating `shared_extra_conf` itself is pointless and it's
being done as a convenient place to mix the objects to get things right
in `shared_config` (confusing).
 1. Does it actually do anything?
- Because `shared_config` starts out as an empty object, my first glance
made me think we we're just updating with an empty object and then just
re-assigning. But because we're in a loop, we actually accumulate the
`shared_extra_conf` from each worker.

I'm not sure whether I'm capturing my confusion well enough here but
basically, this made me spend time trying to figure out what/why we're
doing things this way and we can use a more clear pattern to accomplish
the same thing.

---

This change is spawning from looking at the
`docker/configure_workers_and_start.py` script in order to add a metrics
listener ([upcoming
PR](https://github.com/element-hq/synapse/pull/19324)).

changelog.d/19323.misc

@@ -0,0 +1 @@
+Make it more clear how `shared_extra_conf` is combined in our Docker configuration scripts.

docker/configure_workers_and_start.py

@@ -879,8 +879,8 @@ def generate_worker_files(
         # worker.
         shared_config = {
             **worker_config["shared_extra_conf"],
-            # We combine `shared_config` second to avoid overwriting existing keys
-            # because TODO: why?
+            # We combine `shared_config` second to avoid overwriting existing keys just
+            # for sanity sake (always use the first worker).
             **shared_config,
         }