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

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* [x] Pull request is based on the develop branch
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"Fixed a bug that prevented receiving messages from other servers."
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* [x] [Code
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---------

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

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