docs: Improve examples

[baolsen]

There are some issues with the examples/docs:

• Examples refer to slack_webhook_url or webhook_url which aren't valid parameters
• Examples have a typo - notifcation_schedule
• Examples not well-integrated with TF Cloud (can't choose from dropdown)
• Docs indicate that webhook_urls is required when it isn't
• (optional) and (required) are not consistent on all variables