Docs: illustrate how to process input messages manually

[anton-trunov]

If someone needs to manually process all input messages, they would need to do it in the following manner:

contract MyContract {
    fun rcvWithoutOpcode(msg: Slice) { dump(42) }

    receive(msg: Slice) {
        // load opcode from `msg` first
        rcvWithoutOpcode(msg);
    }

    bounced(msg: Slice) { rcvWithoutOpcode(msg) }
}

Mention also msg in the bounced receiver won't have the 0xFFFFFFFF opcode.

This should be linked where the docs mention the receivers' priorities: