Docs: vtgate does not support replication commands

[lizztheblizz]

For addition to this page: https://vitess.io/docs/12.0/reference/compatibility/mysql-compatibility/

I believe it makes sense to mention that vtgate's SQL interpretation does not support commands related to replication. It can be confusing to users to be in an environment that looks and feels like MySQL, but not having those commands available, and coming to this compatibility page without finding an explanation.