DOCS-727: clarification about sparse indexes

Author: Sam Kleinman

source/core/indexes.txt

@@ -316,7 +316,7 @@ Sparse Index
 ~~~~~~~~~~~~
 
 Sparse indexes only contain entries for documents that have the
-indexed field. By contrast, non-sparse indexes contain all documents
+indexed field. [#null-values-are-indexed-in-sparse-indexes]_ By contrast, non-sparse indexes contain all documents
 in a collection, and store null values for documents that do not
 contain the indexed field. Create a sparse index on the ``xmpp_id``
 field, of the ``members`` collection, using the following operation in
@@ -334,7 +334,7 @@ the :program:`mongo` shell:
 
 .. note::
 
-   Sparse indexes in MongoDB are not to be confused with `block-level`_
+   Do not confuse sparse indexes in MongoDB with `block-level`_
    indexes in other databases. Think of them as dense indexes with a
    specific filter.
 
@@ -345,6 +345,10 @@ the :program:`mongo` shell:
 
    .. _`block-level`: http://en.wikipedia.org/wiki/Index_%28database%29#Sparse_index>
 
+.. [#null-values-are-indexed-in-sparse-indexes] All documents that
+   have the indexed field *are* indexed in a sparse index, even if
+   that field stores a null value in some documents.
+
 .. _index-creation-operations:
 .. _index-operations:
 
@@ -394,7 +398,7 @@ construction:
 
   .. versionchanged:: 2.2
      Before 2.2, a single :program:`mongod` instance could only build
-     one index at a time. 
+     one index at a time.
 
 - The indexing operation runs in the background so that other database
   operations can run while creating the index. However, the