Add note on async invocations for the Kafka connector

[welteki]

Description

Add note on async invocations for the Kafka connector.

Mention the improved support for async invocations with the Kafka connector and link to the Helm docs for configuration instructions.

Motivation and Context

• I have raised an issue to propose this change (required)

Update the docs to reflect the latest changes for the Kafka connector around back-pressure for async invocations.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s