Add additional information about submessage data handling to SEMANTICS.md

[0xekez]

This is a bit of a tricky case for setting data which burned me recently. Contributing some docs in the hope that future CosmWasm people don't need to figure it out themselves.

[kulikthebird]

It's been a while since you posted this PR, but finally we had time to double-check your change. You are right, in today's version wasmd does not forward the data in scenario like A--exec-->B--exec--->C. In this case A won't receive response from C without B forwarding that data explicitly. There's a bigger docs cleanup and this change will be taken into account in CosmWasm/docs#167. Thank you for pointing that out and for your contribution!