Update description of `bounced()` message receiver

[novusnota]

It currently only describes how to accept the raw Slice and the truncated to 224 bits bounced<Message>. But if the message occupies less than 224-bits, than it can be specified there directly, without the special type constructor bounced<…>.

Some prior discussion (and links to implementation in Tact 1.1.0!): https://github.com/tact-lang/tact/issues/29