add upsert section

Author: rachel-mack

source/write/update.txt

@@ -61,6 +61,20 @@ Each update method requires the following parameters:
   :manual:`Field Update Operators guide page</reference/operator/update-field/>` in the
   {+mdb-server+} manual.
 
+Upsert Option
+~~~~~~~~~~~~~
+	
+The ``upsert`` parameter allows you to use the ``update_one()`` and
+``update_many()`` operations to use one method to perform a conditional
+operation:
+
+- If the specified document exists, the command updates it
+- If the specified document does not exist, the command creates a new document
+  with the specified parameter.
+
+For more information, see the :ref:`<ruby-upsert-example>`
+on this page, or the :ref:`<ruby-update-options-table>`.
+
 Update One Document Example
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -117,6 +131,8 @@ operation with default settings.
 The following table describes the options that you can use to
 configure the update operation:
 
+.. _ruby-update-options-table:
+
 .. list-table::
    :widths: 30 70
    :header-rows: 1
@@ -161,6 +177,8 @@ configure the update operation:
          </reference/command/update/#std-label-update-let-syntax>` in the
          {+mdb-server+} manual.
 
+.. _ruby-upsert-example:
+
 Modify Update Example
 `````````````````````