Update note re Maint mode (#876)

Author: Feediver1

modules/manage/pages/node-management.adoc

@@ -14,6 +14,7 @@ When a broker is placed in maintenance mode, if the replication factor is greate
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 * Maintenance mode only transfers leadership. It does not move any partitions to other brokers in the cluster.
 * If a broker hosts a partition with a replica count of one, the partition is unavailable when the Redpanda process is not running.
+* Maintenance mode operations only handle a single broker in maintenance at a time. If you attempt to place more than one broker in maintenance mode, you will get an error.
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 The amount of time it takes to drain a broker and reassign leadership depends on the number of partitions and the health of the cluster. For healthy clusters, draining leadership should take less than a minute. For unhealthy clusters (for example, when follower is not in sync with the leader), draining the broker can take longer. Note that the draining process won't start until the cluster is healthy.