Update field_types.rst

I thought that it would be good to mention here that all symfony 2 field types are accepted.
http://symfony.com/doc/current/reference/forms/types.html

textarea as example

Resources/doc/reference/field_types.rst

@@ -11,6 +11,7 @@ There are many field types that can be used in the list action or show action :
 * **date**: display a formatted date. Accepts an optional ``format`` parameter
 * **datetime**: display a formatted date and time. Accepts an optional ``format`` parameter
 * **text**: display a text
+* **textarea**: display a textarea
 * **trans**: translate the value with a provided ``catalogue`` option
 * **string**: display a text
 * **decimal**: display a number