Added @api tag for documentation

src/Spout/Reader/XLSX/Sheet.php

@@ -36,6 +36,7 @@ class Sheet implements SheetInterface
     }
 
     /**
+     * @api
      * @return RowIterator
      */
     public function getRowIterator()
@@ -44,6 +45,7 @@ class Sheet implements SheetInterface
     }
 
     /**
+     * @api
      * @return int Index of the sheet, based on order of creation (zero-based)
      */
     public function getIndex()
@@ -52,6 +54,7 @@ class Sheet implements SheetInterface
     }
 
     /**
+     * @api
      * @return string Name of the sheet
      */
     public function getName()