Improve Federation Trusted Server Documentation

Is your feature request related to a problem? Please describe.
my two NC servers would not sync. addressbooks

Describe the solution you'd like
A) please add more details to the documentation on how this works

for example a simple flow diagram (and links to the specs)

• cloudA: (as documented) add URL of cloudB to Settings/Sharing/Trusted servers
• cloudA: tell cloudB to get a shared secret "sudo -u www-data php -f cron.php"
• cloudB: get shared secret "sudo -u www-data php -f cron.php"
• cloudA: sync addressbook "sudo -u www-data php occ federation:sync-addressbooks"

Besides a more technical description of the feature, some questions are not answered by the documentation

• will "federation:sync-addressbooks" be called eventually, if so when? can the intervall be configured?
• will changes be synched automatically?

B) Enhancement
apps/federation/lib/SyncFederationAddressBooks.php
instead of skipping problems with the shared_secret

      if (is_null($sharedSecret)) {
        continue;
      }

a warning should be logged, possibly with instructions on how to get the secret.

Related issues
nextcloud/server#14231, nextcloud/server#6349

Review the existing federation trusted-server and address-book documentation, then inspect apps/federation/lib/SyncFederationAddressBooks.php and the related issues. Document the setup flow with a diagram and specification links, answer the synchronization-scheduling questions, and define the missing shared-secret warning behavior. Done means the requested documentation details and actionable error guidance are covered.