Fix docs for message.key.columns and skipped.operations

io.debezium.config.CommonConnectorConfig#SKIPPED_OPERATIONS states that `i` would be the correct value for INSERT events, but that's not correct. It should be io.debezium.data.Envelope.Operation#CREATE which is a `c`.
see: https://github.com/debezium/debezium/blob/master/debezium-core/src/main/java/io/debezium/config/CommonConnectorConfig.java#L360

The docs for message.key.columns state the default value would be an empty string. this is not correct. null/unset is the default and thus should be replaced with: n/a