clarify that CMS defaults change is coming in 1.2

docs/reference/index-modules/merge.asciidoc

@@ -201,9 +201,14 @@ The scheduler supports the following settings:
 
 `index.merge.scheduler.max_thread_count`::
 
-The maximum number of concurrent merge threads that may run at once. Defaults
-to `1` which works best with spinning-magnets disks.  If you are using
-a good solid-state disk (SSD) instead then try setting this to `3`.
+The maximum number of threads to perform the merge operation. Defaults to
+`Math.max(1, Math.min(3, Runtime.getRuntime().availableProcessors() / 2))`.
+
+coming[1.2.0]
+
+The default will change to `1` which works best with spinning-magnets
+disks.  If you are using a good solid-state disk (SSD) instead then try
+setting this to `3`.
 
 [float]
 ==== SerialMergeScheduler