Update master

Author: matulef

draft/core/indexes.txt

@@ -31,8 +31,8 @@ indexes:
   :ref:`compound index <index-type-compound>`.
 
 - Queries that are "covered" by the index return more quickly
-  than queries that have to scan many individual documents. An index is 
-  considered to "cover" a query if all the data that the query must return
+  than queries that have to scan many individual documents. An index
+  "covers" a query if all the data that the query must return
   is stored in within the keys of the index.  
 
 - Using queries with good index coverage will reduce the number of full 
@@ -69,7 +69,7 @@ _id
 
 The ``_id`` index is a :ref:`unique index <index-type-unique>`
 [#unique-index-report]_ on the ``_id`` field, and MongoDB creates this
-index by default on all collections. [#capped-collections]_ You cannot
+index by default on all collections (except for [#capped-collections]). You cannot
 delete the index on ``_id``.
 
 The ``_id`` field is the :term:`primary key` for the collection, and
@@ -80,17 +80,20 @@ are 12-byte, unique identifiers, that make suitable ``_id`` values.
 
 .. note::
 
-   In :term:`shard clusters <shard cluster>`, if the you do *not* use
+   In :term:`shard clusters <shard cluster>`, if you do *not* use
    the ``_id`` field as the :term:`shard key`, then your application
    **must** ensure the uniqueness of the values in the ``_id`` field
-   to prevent errors.
+   to prevent errors.  This is most-often done by using the standard
+   auto-generated :term:`ObjectIds`. 
 
 .. [#unique-index-report] Although the index on ``_id`` *is* unique,
    the :func:`getIndexes() <db.collection.getIndexes()>` method will
    *not* print ``unique: true`` in the :program:`mongo` shell.
 
-.. [#capped-collections] Capped collections are a special collection
-   which do not have an ``_id`` index.
+.. [#capped-collections] Capped collections are special collections
+   which do not have an ``_id`` index by default.  
+   TODO: figure out what new behavior of capped collections is.
+   (i think in replset capped collections now have _id by default)
 
 .. _index-types-secondary:
 
@@ -109,9 +112,9 @@ primary, common, and user-facing queries and require MongoDB to scan
 the fewest number of documents possible.
 
 To create a secondary index, use the :func:`ensureIndex()`
-method. The specifications an index using the :func:`ensureIndex()
-<db.collection.ensureIndex()>` operation will resemble the following
-on the MongoDB shell:
+method. The argument to :func:`ensureIndex()
+<db.collection.ensureIndex()>` will resemble the following
+in the MongoDB shell:
 
 .. code-block:: javascript
 
@@ -142,7 +145,7 @@ documents that resemble the following example document:
              }
    }
 
-You could create an index on the ``address.zipcode`` field, using the
+You can create an index on the ``address.zipcode`` field, using the
 following specification:
 
 .. code-block:: javascript
@@ -160,7 +163,7 @@ Compound Indexes
 MongoDB supports "compound indexes," where a single index structure
 holds references to multiple fields within a collection's
 documents. Consider the collection ``products`` that holds documents
-that resemble the following an example document:
+that resemble the following  example document:
 
 .. code-block:: javascript