added to pre-splits entry in glossary

Author: Bob Grabar

source/reference/glossary.txt

@@ -512,11 +512,13 @@ Glossary
       MongoDB will return from the documents in the result set.
 
    pre-splitting
-      When deploying a :term:`shard cluster`, it is sometimes
-      necessary to expedite the initial distribution of documents
-      among shards by manually dividing the collection into chunks.
-      In this case, you pre-split the chunks instead of waiting for
-      the MongoDB :term:`balancer` to create splits.
+      Pre-splitting is a command run prior to data insertion that
+      specifies the shard key values on which to split up chunks.
+      When deploying a :term:`shard cluster`, it is sometimes necessary
+      to expedite the initial distribution of documents among shards by
+      manually dividing the collection into chunks. In this case, you
+      pre-split the chunks instead of waiting for the MongoDB
+      :term:`balancer` to create splits.
 
    SSD
       Solid State Disk. A high-performance disk drive that uses solid