Refer to 'mask' instead of 'block' in documentation of Control.Exception

Hello, Attached is a small documentation fix for Control.Exception that replaces references to the deprecated 'block' with the recommended 'mask' combinator. There's one line in the section 'Applying mask to an exception handler' that I'm not sure about what to do with: "If you need to unblock asynchronous exceptions again in the exception handler, just use 'unblock' as normal." Do we really need to point users to a deprecated function? Regards, Bas