DOCS-16428-upsert-n (#5074) (#5094)

* DOCS-16428-upsert-n

* DOCS-16428-upsert-n

* DOCS-16428-upsert-n

* DOCS-16428-upsert-n

* DOCS-16428-upsert-n

---------

Co-authored-by: jason-price-mongodb <[email protected]>

Author: jason-price-mongodb

source/reference/command/update.txt

@@ -1140,7 +1140,13 @@ The returned document contains a subset of the following fields:
 
 .. data:: update.n
 
-   The number of documents selected for update. If the update operation
+   An ``update`` command accepts an array of document updates, some of
+   which can be :term:`upserts <upsert>`. For an update, ``n`` is the
+   number of documents selected for the update. For an upsert, ``n`` is
+   ``1`` for the inserted document. The server adds the ``n`` values for
+   all the updates and upserts and returns the total as ``update.n``.
+
+   If an update operation
    results in no change to the document, e.g. :update:`$set` expression
    updates the value to the current value, :data:`~update.n` can be
    greater than :data:`~update.nModified`.