Add documentation on what each parameter to a `sla_miss_callback` callable is

[CrossNox]

I couldn't find any official documentation specifying what each parameter to a sla_miss_callback callable are. This would be a great addition to know how to properly format the messages sent.

[boring-cyborg]

Thanks for opening your first issue here! Be sure to follow the issue template!

[vikramkoka]

Thank you for reporting this.
Here is the high level description for the SLA callback in the docs http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/concepts/tasks.html#slas

Would you like to create a PR to add this?

[alex-astronomer]

I'd love to create a PR for this. This will be my first contribution to Airflow. I searched for "good first issue" and "documentation" labels for my first try at this. Would this Issue be a good place to start making contributions?

[potiuk]

Sure. Feel free!