Schemarouter monitoring requirements are not documented clearly

When we have a Sharding Cluster with SchemaRouter we could come to the idea to drain/set to maintenance a Shard (for example for upgrade the db or the server including a reboot). For this I would have expected to work drain/maintenance.
maxctrl list servers
┌────────┬──────────────┬──────┬─────────────┬─────────────────┬──────────────┬─────────────────┐
│ Server │ Address │ Port │ Connections │ State │ GTID │ Monitor │
├────────┼──────────────┼──────┼─────────────┼─────────────────┼──────────────┼─────────────────┤
│ shard2 │ 10.139.158.1 │ 3364 │ 1 │ Master, Running │ 0-3364-69817 │ MariaDB-Monitor │
├────────┼──────────────┼──────┼─────────────┼─────────────────┼──────────────┼─────────────────┤
│ shard3 │ 10.139.158.1 │ 3365 │ 1 │ Running │ 0-3365-63166 │ MariaDB-Monitor │
├────────┼──────────────┼──────┼─────────────┼─────────────────┼──────────────┼─────────────────┤
│ shard4 │ 10.139.158.1 │ 3366 │ 1 │ Running │ 0-3366-6902 │ MariaDB-Monitor │
└────────┴──────────────┴──────┴─────────────┴─────────────────┴──────────────┴─────────────────┘

maxctrl set server shard2 drain
maxctrl set server shard2 maintenance

But I am getting an error:

Error: Server at http://127.0.0.1:8989 responded with 400 Bad Request to `PUT servers/shard2/set?state=drain`
{
"errors": [

{ "detail": "The server is primary, so it cannot be set in maintenance or draining mode. First perform a switchover and then retry the operation." }

]
}

As I wrote before I do not understand the concept of "Master" in a sharding system.

So when I try to "switchover" the master role to another shard I get the following error:

maxctrl call command mariadbmon switchover MariaDB-Monitor shard3 shard2
Error: Server at http://127.0.0.1:8989 responded with 400 Bad Request to `POST maxscale/modules/mariadbmon/switchover?MariaDB-Monitor&shard3&shard2`
{
"links":

{ "self": "http://127.0.0.1:8989/v1/maxscale/modules/mariadbmon/switchover/" }

,
"meta": {
"errors": [

{ "detail": "'shard3' is not a valid promotion target for switchover because it is not replicating from 'shard2'." }

,

{ "detail": "Switchover cancelled." }

]
}
}

So I am a bit clueless now, how I should proceed...

@Markus: I wrote everything down for a blog article, if you want to cross-read before publishing, please let me know...