client: Update docs

[dveeden]

1. Add comments to GetServerVersion and CompareServerVersion.
2. Move examples to a separate file.
   • godoc can find them there and do proper formatting etc.
   • As no expected output is included and these can't be run individually, this is not yet a testable example
   • This fixed the formatting (old formatting can be seen on https://pkg.go.dev/github.com/go-mysql-org/go-mysql@v1.10.0/client#Conn.ExecuteMultiple )