DOCS-896 section about document growth in write operations

Author: Sam Kleinman

source/core/write-operations.txt

@@ -342,12 +342,32 @@ write operation that affects multiple documents using the
 To isolate a sequence of write operations from other read and write
 operations, see :doc:`/tutorial/perform-two-phase-commits`.
 
+Updates
+-------
+
+Each document in a MongoDB collection has allocated *record space*
+which includes the entire document *and* a small amount of
+padding. This padding makes it possible for update operations to
+increase the size of a document slightly without causing the document
+to outgrow the allocated record size. 
+
+Documents in MongoDB can grow up to the full maximum :limit:`BSON
+document size <BSON Document Size>`; however, when documents outgrow
+their record size MongoDB must allocate a new record and move the
+document to the new record. Update operations that do not cause a
+document to grow, (i.e. *in-place* updates,) are significantly more
+efferent than those updates that cause document growth. Use :doc:`data
+models </core/data-modeling>` that minimize the need for document
+growth when possible.
+
+For complete examples of update operations, see
+:doc:`/applications/update`. 
+
 .. _write-operations-padding-factor:
 
 Padding Factor
 --------------
 
-
 If an update operation does not cause the document to increase in
 size, MongoDB can apply the update in-place. Some updates
 change the size of the document, for example using the