indexing: minor edit

source/applications/indexes.txt

@@ -152,8 +152,8 @@ create a sorted index on that field.
 
 To sort query results on multiple fields, create a :ref:`compound index
 <index-type-compound>`. MongoDB sorts results based on the field order
-in the index. Ensure that any index constraints on index fields that are
-before the first sort field are equalities.
+in the index. Ensure that any index fields before the first sort field
+are equalities.
 
 .. example::
 

source/faq/indexes.txt

@@ -83,11 +83,10 @@ documentation on choosing which fields to index, see
 How do write operations affect indexes?
 ---------------------------------------
 
-While a read operation does not affect an index, every write operation
-does involve a write to the index. If your application is write-heavy,
-creating too many indexes might affect performance.
-
-.. todo:: More is needed on that last FAQ.
+While a read operation on an indexed key does not affect indexes, any
+write operation to an indexed key involves writing not only to the
+document but also to the appropriate index. If your application is
+write-heavy, creating too many indexes might affect performance.
 
 Will building a large index affect database performance?
 --------------------------------------------------------