Finalize IBC Events

[ethanfrey]

These are not yet defined in EVENTS.md.
They were not critical to finalise for 0.18, as other cosmwasm contracts cannot listen to those (so they are not contract API breaking), but it is good to document and stabilise those before a 1.0 release

[ethanfrey]

On reflection, contracts will never execute the ibc entry points except in very contrived examples.

The ibc handlers already have well-defined events for IBC messages (which are used by the relayers to work). We can just reference those events and not generate a specific cosmwasm event. You can already query events on port_id and/or channel_id which should be enough to track a contract.

Unless there are other comments, I would just fix the doc to reference standard IBC