Avoid rotating pods for PGVERSION change outside of maintenance window (#3065)

* Avoid rotating pods for PGVERSION change outside of maintenance window
* Update docs

docs/administrator.md

@@ -65,7 +65,10 @@ the `PGVERSION` environment variable is set for the database pods. Since
 In-place major version upgrades can be configured to be executed by the
 operator with the `major_version_upgrade_mode` option. By default, it is
 enabled (mode: `manual`). In any case, altering the version in the manifest
-will trigger a rolling update of pods to update the `PGVERSION` env variable.
+will update the desired `PGVERSION`. If `maintenanceWindows` are configured,
+major-version-related pod rotation is deferred until the next maintenance
+window. Without maintenance windows, the operator will trigger a rolling
+update of pods to apply the new `PGVERSION`.
 Spilo's [`configure_spilo`](https://github.com/zalando/spilo/blob/master/postgres-appliance/scripts/configure_spilo.py)
 script will notice the version mismatch but start the current version again.
 
@@ -93,8 +96,9 @@ Thus, the `full` mode can create drift between desired and actual state.
 ### Upgrade during maintenance windows
 
 When `maintenanceWindows` are defined in the Postgres manifest the operator
-will trigger a major version upgrade only during these periods. Make sure they
-are at least twice as long as your configured `resync_period` to guarantee
+will trigger major-version-related pod rotation and the major version upgrade
+only during these periods. Make sure they are at least twice as long as your
+configured `resync_period` to guarantee
 that operator actions can be triggered.
 
 ### Upgrade annotations