DOCS-14301 address chunk balancing changes

Author: npentrel

source/reference/command/split.txt

@@ -15,13 +15,17 @@ Definition
 
 .. dbcommand:: split
 
-   Splits a :term:`chunk` in a :term:`sharded cluster` into two
-   chunks. The :binary:`~bin.mongos` instance splits and manages
-   chunks automatically, but for exceptional circumstances the
-   :dbcommand:`split` command does allow administrators to manually
-   create splits. See :doc:`/tutorial/split-chunks-in-sharded-cluster` for
-   information on these circumstances, and on the MongoDB shell commands
-   that wrap :dbcommand:`split`.
+   .. versionchanged:: 4.2
+
+   Splits a :term:`chunk` in a :term:`sharded cluster` into two chunks.
+   Starting in MongoDB 4.2, shards manage and split chunks based on the
+   chunk size statistics they maintain.
+
+   For exceptional circumstances the :dbcommand:`split` command does
+   allow administrators to manually create splits. See
+   :doc:`/tutorial/split-chunks-in-sharded-cluster` for information on
+   these circumstances, and on the MongoDB shell commands that wrap
+   :dbcommand:`split`.
 
    The :dbcommand:`split` command must be run in the ``admin`` database
    and uses the following form: