Update doc block for 'keepExisting' HABTM option.

lib/Cake/Model/Model.php

@@ -447,6 +447,7 @@ class Model extends Object implements CakeEventListener {
  * - `unique`: If true (default value) cake will first delete existing relationship
  *   records in the foreign keys table before inserting new ones, when updating a
  *   record. So existing associations need to be passed again when updating.
+ *   To prevent deletion of existing relationship records, set this key to a string 'keepExisting'.
  * - `conditions`: An SQL fragment used to filter related model records. It's good
  *   practice to use model names in SQL fragments: "Comment.status = 1" is always
  *   better than just "status = 1."