Extend the doc block to explain the possibility to return null to uns…

…et filter

Basically an extension due to the discussion in Smile-SA#1158. Since one does not
need to return a \Smile\ElasticsuiteCore\Search\Request\QueryInterface the
doc block comment should indicate that.

src/module-elasticsuite-core/Api/Search/Request/Container/FilterInterface.php

@@ -24,11 +24,11 @@
 interface FilterInterface
 {
     /**
-     * Get filter query according to current search context.
+     * Get filter query according to current search context. Return null to unset filter query.
      *
      * @param \Smile\ElasticsuiteCore\Api\Search\ContextInterface $searchContext Search Context
      *
-     * @return \Smile\ElasticsuiteCore\Search\Request\QueryInterface
+     * @return \Smile\ElasticsuiteCore\Search\Request\QueryInterface|null
      */
     public function getFilterQuery(\Smile\ElasticsuiteCore\Api\Search\ContextInterface $searchContext);
 }