DOCS-584: style corrections

Author: Sam Kleinman

source/reference/glossary.txt

@@ -512,13 +512,14 @@ Glossary
       MongoDB will return from the documents in the result set.
 
    pre-splitting
-      Pre-splitting is an action you can perform prior to data insertion
-      that specifies the shard key values on which to split 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.
+      An operation, performed before inserting data that divides the
+      range of possible shard key values into chunks to facilitate
+      easy insertion and high write throughput.  When deploying a
+      :term:`shard cluster`, in some cases pre-splitting will expedite
+      the initial distribution of documents among shards by manually
+      dividing the collection into chunks rather than waiting for the
+      MongoDB :term:`balancer` to create chunks during the course of
+      normal operation.
 
    SSD
       Solid State Disk. A high-performance disk drive that uses solid