Update the adaptive scheduler doc about batch job limitation

FLINK-30846 [1] updated the fall back strategy from default Scheduler to AdaptiveBatch Scheduler when batch job enable `jobmanager.scheduler: adaptive`.

However, the doc[2] isn't updated.

[1] https://github.com/apache/flink/pull/21814/files
[2] https://nightlies.apache.org/flink/flink-docs-master/docs/deployment/elastic_scaling/#limitations-1