Fix doc blocks & types (mghoneimy#51)

Improve doc blocks and type checking after PHPStan analysis

Author: GenieTim

src/QueryBuilder/AbstractQueryBuilder.php

@@ -72,7 +72,7 @@ public function getQuery(): Query
 
         // Convert nested query builders to query objects
         foreach ($this->selectionSet as $key => $field) {
-            if ($field instanceof AbstractQueryBuilder) {
+            if ($field instanceof QueryBuilderInterface) {
                 $this->selectionSet[$key] = $field->getQuery();
             }
         }
@@ -85,15 +85,15 @@ public function getQuery(): Query
     }
 
     /**
-     * @param string|QueryBuilder|Query $selectedField
+     * @param string|QueryBuilderInterface|InlineFragment|Query $selectedField
      *
      * @return $this
      */
     protected function selectField($selectedField)
     {
         if (
             is_string($selectedField)
-            || $selectedField instanceof AbstractQueryBuilder
+            || $selectedField instanceof QueryBuilderInterface
             || $selectedField instanceof Query
             || $selectedField instanceof InlineFragment
         ) {
@@ -132,4 +132,4 @@ protected function setVariable(string $name, string $type, bool $isRequired = fa
 
         return $this;
     }
-}
+}