Add information regarding the DLQ from the source connector

Description

URL: https://docs.mongodb.com/kafka-connector/current/kafka-source/

For the configuration errors.deadletterqueue.topic.name update the description to let the user know that errors are sent to the topic as extended json strings.

A/C

As a reader, I understand that errors are sent to the deadletter queue topic as an extended JSON string.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)