Update SEMANTICS.md

[Kayanski]

The SEMANTICS.md file is very convenient to understand how executions, replies and submessages deal with the Response fields (events, data).

Cosmwasm 2.x introduced a new msg_responses field that is NOT documented in the SEMANTICS.md file.

Is it possible to update this file to have an explanation of how that field is handled during execution ?
And more generally, is it possible to update this file to 2.x specifics ?

Thanks

[chipshort]

Hey Kayanski,

Yes, the whole document is very outdated.
The msg_responses field just gets forwarded from the SDK, so maybe more information can be found there. x/wasm just takes what it gets from the SDK and combines it into one vec. The only thing it does different is the fact that one message from a contract can be translated into multiple sdk messages (does not happen for any of the preimplemented messages, but could be the case for custom messages), so x/wasm concatenates all the MsgResponses fields from all the message responses into one vec.

Thanks for bringing this point up.