Fix empty_data option description of TextareaType

Author: chapa

reference/forms/types/textarea.rst

@@ -35,7 +35,9 @@ These options inherit from the :doc:`FormType </reference/forms/types/form>`:
 
 .. include:: /reference/forms/types/options/empty_data_declaration.rst.inc
 
-The default value is ``''`` (the empty string).
+From an HTTP perspective, submitted data is always a string or an array of strings.
+So by default, the form will treat any empty string as null. If you prefer to get
+an empty string, explicitly set the ``empty_data`` option to an empty string.
 
 .. include:: /reference/forms/types/options/empty_data_description.rst.inc