Update source/administration/sharding.txt

Per DOCS-983 - explicit warning

Author: comerford

source/administration/sharding.txt

@@ -595,6 +595,14 @@ insert operation, and can only migrate a single chunk at a time. Chunk
 migrations are resource intensive and further complicated by large
 write volume to the migrating chunk.
 
+   .. warning::
+
+      Pre-splitting can only be performed on an empty collection.
+      When you enable sharding for a collection containing data the
+      collection will be automatically split for you.  Subsequent
+      attempts to pre-split as below may lead to strange and 
+      unpredictable chunk ranges and sizes.
+
 To create and migrate chunks manually, use the following procedure:
 
 #. Split empty chunks in your collection by manually performing