[DOCS] Update merge docs to reflect the max_merge_at_once property

Author: s1monw

docs/reference/index-modules/merge.asciidoc

@@ -209,4 +209,16 @@ The maximum number of threads to perform the merge operation. Defaults to
 
 A merge scheduler that simply does each merge sequentially using the
 calling thread (blocking the operations that triggered the merge or the
-index operation).
+index operation). This merge scheduler has a merge thread pool that
+explicitly schedules merges, and it makes sure that merges are serial
+within a shard, yet concurrent across multiple shards.
+
+The scheduler supports the following settings:
+
+`index.merge.scheduler.max_merge_at_once`::
+
+The maximum number of merges a single merge run performs. This setting prevents
+executing unlimited amount of merges in a loop until another shards has a
+chance to get a merge thread from the pool. If this limit is reached the
+merge thread returns to the pool and continues once the the call to a single
+shards is executed. The default is `5`