DOCS-14228 Clarified updateExisting functionality

Author: ianf-mongodb

source/reference/command/findAndModify.txt

@@ -325,7 +325,11 @@ The ``lastErrorObject`` embedded document contains the following fields:
 
      - boolean
 
-     - Contains ``true`` if an ``update`` operation modified an existing document.
+     - Contains ``true`` if an ``update`` operation:
+
+       - Modified an existing document.
+       - Found the document, but it was already in the
+         desired destination state so no update actually occurred.