Various small updates to the federation documentation

[supersven]

After I've configured federation for another set of envs, I took the time to revise our documentation about it a bit.

Checklist

• [X] The PR Title explains the impact of the change.
• [X] The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• [X] changelog.d contains the following bits of information (details):
  • [X] A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[CLAassistant]

All committers have signed the CLA.

[julialongtin]

@supersven can you fix the conflicts on this so we can merge it?