Trim down date/time hint types.

While other types are accepted, having many different types, which are
usually only used when _setting_ data, might be confusing especially
for beginners.

src/View/Helper/DocBlockHelper.php

@@ -110,7 +110,7 @@ public function columnTypeToHintType($type)
             case 'datetime':
             case 'time':
             case 'timestamp':
-                return 'string|int|\DateTime|\Cake\I18n\Time';
+                return '\Cake\I18n\Time';
         }
 
         return null;