explain load balancing for federation_sender_instances (#17776)

Adding information on how the load is distributed for
`federation_sender_instances`.

Thx to @devonh for the information.

causal source:
https://github.com/element-hq/synapse/blob/c2e5e9e67c24264f5a12bf3ceaa9c4e195547d26/synapse/config/_base.py#L946-L989

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Devon Hudson <devon.dmytro@gmail.com>

changelog.d/17776.doc

@@ -0,0 +1 @@
+Explain how load balancing works for `federation_sender_instances`.

docs/usage/configuration/config_documentation.md

@@ -4368,7 +4368,13 @@ It is possible to scale the processes that handle sending outbound federation re
 by running a [`generic_worker`](../../workers.md#synapseappgeneric_worker) and adding it's [`worker_name`](#worker_name) to
 a `federation_sender_instances` map. Doing so will remove handling of this function from
 the main process. Multiple workers can be added to this map, in which case the work is
-balanced across them.
+balanced across them. 
+
+The way that the load balancing works is any outbound federation request will be assigned 
+to a federation sender worker based on the hash of the destination server name. This
+means that all requests being sent to the same destination will be processed by the same
+worker instance. Multiple `federation_sender_instances` are useful if there is a federation
+with multiple servers.
 
 This configuration setting must be shared between all workers handling federation
 sending, and if changed all federation sender workers must be stopped at the same time