Add in docmentation on setting value of multiselect custom value

docs/api/custom-data.md

@@ -64,4 +64,10 @@ For entities other than the Contact Entity you can use an alternate notation to
 !!! note 
     When retrieving custom data for contact entity, it will only return one value in the case of a multiple custom group set whereas for other entities (e.g. Address, or using the `CustomValue.get` API) you will get all custom data records that relate to the relevant entity
 
-The CustomValue Entity implicitly determines what the `entity_table` variable should be when it is not supplied. If you find that the implicitly is not working out exactly, then specify the `entity_table` key. 
+The CustomValue Entity implicitly determines what the `entity_table` variable should be when it is not supplied. If you find that the implicitly is not working out exactly, then specify the `entity_table` key.
+
+When setting the value of custom data that is of type checkbox or multivalue it is important to note that the options need to be passed in as an array. For example if i want to set options a and c of for custom value 2 on contact you should do the following
+
+```php
+$result = civicrm_api3('Contact', 'create', array('id' = 2, 'custom_2' => array('a', 'c'));
+``` `