DOCS-831 review edits

Author: Bob Grabar

source/core/sharding-internals.txt

@@ -537,27 +537,31 @@ returned document does not include the ``isdbgrid`` string.
 Shard GridFS Documents
 ----------------------
 
-One common way to shard :term:`GridFS` is to do so based on pre-existing
-indexes and to configure the shard as follows:
+A common way to shard :term:`GridFS` is to configure the shard as follows:
 
-- Do not shard the "files" collection. This means all the file-metadata
-  documents live on one shard. It is highly recommended that the shard
-  is a replica set with at least three members, for resiliency.
+- Do not shard the ``files`` collection, as the keys in this collection do
+  not easily lend themselves to even distributions.
 
-- Shard the "chunks" collection using the index "files_id: 1". You must
-  create this separate index. Do not use the existing "files_id, n"
-  index created by the drivers.
+  Leaving ``files`` unsharded means that all the file metadata documents
+  live on one shard. It is recommended that the shard is a replica set
+  with at least three members, for high availability.
 
-  The new "files_id" index ensures that all chunks of a given file live
-  on the same shard, which is safer and allows FileMD5 hashing.
+- Shard the ``chunks`` collection using a new ``files_id : 1 , n : 1``
+  index. You must create this index. Do not use the existing
+  ``files_id : 1 , n : 1`` index already created by the drivers.
 
-  To shard "chunks" by "files_id", issue commands similar to the following:
+  The new ``files_id : 1 , n : 1`` index ensures that all chunks of a
+  given file live on the same shard, which is safer and allows FileMD5
+  hashing.
+
+  To shard the ``chunks`` collection by ``files_id : 1 , n : 1``, issue
+  commands similar to the following:
 
   .. code-block:: javascript
 
-     db.fs.chunks.ensureIndex( { files_id : 1 } )
+     db.fs.chunks.ensureIndex( { files_id : 1 , n : 1 } )
 
-     db.runCommand( { shardcollection : "test.fs.chunks" , key : { files_id : 1 } } )
+     db.runCommand( { shardcollection : "test.fs.chunks" , key : { files_id : 1 , n : 1 } } )
 
   The default ``files_id`` is an :term:`ObjectId`. The ``files_id`` is
   ascending, and all GridFS chunks are sent to a single sharding chunk.