Note that forcemerges now run in parallel in docs (#69688)

Relates #69416

docs/reference/indices/forcemerge.asciidoc

@@ -68,11 +68,15 @@ You can force merge multiple indices with a single request by targeting:
 * One or more index aliases that point to multiple indices
 * All data streams and indices in a cluster
 
-Multi-index operations are executed one shard at a
-time per node. Force merge makes the storage for the shard being merged
-temporarily increase, up to double its size in case `max_num_segments` parameter
-is set to `1`, as all segments need to be rewritten into a new one.
-
+Each targeted shard is force-merged separately using <<modules-threadpool,the
+`force_merge` threadpool>>. By default each node only has a single
+`force_merge` thread which means that the shards on that node are force-merged
+one at a time. If you expand the `force_merge` threadpool on a node then it
+will force merge its shards in parallel.
+
+Force merge makes the storage for the shard being merged temporarily
+increase, up to double its size in case `max_num_segments` parameter is set to
+`1`, as all segments need to be rewritten into a new one.
 
 [[forcemerge-api-path-params]]
 ==== {api-path-parms-title}