Update documentation for Model::_readDataSource()

Update sample code to generate unique keys based on association
conditions as well. Because association conditions are not part of the
'query' they need to be handled separately.

Refs #8346

lib/Cake/Model/Model.php

@@ -3011,15 +3011,15 @@ public function find($type = 'first', $query = array()) {
  *
  * ```
  * protected function _readDataSource($type, $query) {
- * 		$cacheName = md5(json_encode($query));
- * 		$cache = Cache::read($cacheName, 'cache-config-name');
- * 		if ($cache !== false) {
- * 			return $cache;
- * 		}
- *
- * 		$results = parent::_readDataSource($type, $query);
- * 		Cache::write($cacheName, $results, 'cache-config-name');
- * 		return $results;
+ *     $cacheName = md5(json_encode($query) . json_encode($this->hasOne) . json_encode($this->belongsTo));
+ *     $cache = Cache::read($cacheName, 'cache-config-name');
+ *     if ($cache !== false) {
+ *         return $cache;
+ *     }
+ *
+ *     $results = parent::_readDataSource($type, $query);
+ *     Cache::write($cacheName, $results, 'cache-config-name');
+ *     return $results;
  * }
  * ```
  *