Added @api tag for documentation

src/Spout/Writer/Exception/InvalidColorException.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\Exception;
 /**
  * Class InvalidColorException
  *
+ * @api
  * @package Box\Spout\Writer\Exception
  */
 class InvalidColorException extends WriterException

src/Spout/Writer/Exception/InvalidSheetNameException.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\Exception;
 /**
  * Class InvalidSheetNameException
  *
+ * @api
  * @package Box\Spout\Writer\Exception
  */
 class InvalidSheetNameException extends WriterException

src/Spout/Writer/Exception/SheetNotFoundException.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\Exception;
 /**
  * Class SheetNotFoundException
  *
+ * @api
  * @package Box\Spout\Writer\Exception
  */
 class SheetNotFoundException extends WriterException

src/Spout/Writer/Exception/WriterAlreadyOpenedException.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\Exception;
 /**
  * Class WriterAlreadyOpenedException
  *
+ * @api
  * @package Box\Spout\Writer\Exception
  */
 class WriterAlreadyOpenedException extends WriterException

src/Spout/Writer/Exception/WriterNotOpenedException.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\Exception;
 /**
  * Class WriterNotOpenedException
  *
+ * @api
  * @package Box\Spout\Writer\Exception
  */
 class WriterNotOpenedException extends WriterException