Improve allocation-disabling instructions (#30248)

Clarify the “one minute” in the instructions to disable the shard allocation
when doing maintenance to say that it is configurable.

docs/reference/upgrade/disable-shard-alloc.asciidoc

@@ -1,8 +1,10 @@
 
-When you shut down a node, the allocation process waits for one minute
-before starting to replicate the shards on that node to other nodes
-in the cluster, causing a lot of wasted I/O. You can avoid racing the clock
-by disabling allocation before shutting down the node:
+When you shut down a node, the allocation process waits for
+`index.unassigned.node_left.delayed_timeout` (by default, one minute) before
+starting to replicate the shards on that node to other nodes in the cluster,
+which can involve a lot of I/O.  Since the node is shortly going to be
+restarted, this I/O is unnecessary. You can avoid racing the clock by disabling
+allocation before shutting down the node:
 
 [source,js]
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