patching SQL Server with Availability Group and Secondary Replica

The scenario is called out and supported on the link you've provided.

Availability Group with One Remote Secondary Replica

If you have deployed an availability group only for disaster recovery, you may need to fail over the availability group to an asynchronous-commit secondary replica. Such configuration is illustrated by the following figure:

Availability Group Upgrade in DR Scenario

In this situation, you must fail over the availability group to the asynchronous-commit secondary replica during the rolling upgrade/update. To prevent data loss, change the commit mode to synchronous commit and wait for the secondary replica to be synchronized before you fail over the availability group. Therefore, the rolling upgrade/update process may look as follows:

1.Upgrade/update the remote server

2.Change the commit mode to synchronous commit

3.Wait until synchronization state is SYNCHRONIZED

4.Fail over the availability group to the remote site

5.Upgrade/update the local (primary site) server

6.Fail over the availability group to the primary site

7.Change the commit mode to asynchronous commit

The documentation was updated since, and I feel the below is a perfect answer to my scenario:

1. Remove automatic failover on all synchronous-commit replicas

2. Upgrade all remote secondary replica instances running asynchronous-commit secondary replicas

3. Upgrade the all local replica secondary instances that are not currently running the primary replica

4. Manually fail over the AG to a local synchronous-commit secondary replica

5. Upgrade or update the local replica instance that formerly hosted the primary replica

6. Configure automatic failover partners as desired

If necessary, you can perform an extra manual failover to return the AG to its original configuration.