DOCS-249-related: change to write concern glossary entry

Author: Bob Grabar

source/reference/glossary.txt

@@ -588,10 +588,11 @@ Glossary
       nodes have :term:`eventual consistency`.
 
    write concern
-      A setting on writes to MongoDB that allows the user to specify,
-      how the database will handle a write operation before
-      returning. This often determines how many :term:`replica set`
-      members should propagate a write before returning.
+      Specifies whether a write operation has succeeded. Write concern
+      allows your application to detect insertion errors or unavailable
+      :program:`mongod` instances. For :term:`replica sets <replica
+      set>`, you can configure write concern to confirm replication to a
+      specified number of members.
 
       .. seealso:: :ref:`Write Concern for Replica Sets <replica-set-write-concern>`.