Fix issue in IRIS docs and alerter required options

[malinkinsa]

Description

I found some inaccuracies in the description, which have been corrected. I misunderstood the parameter slightly when adding support. After these changes, it doesn't need to be mandatory anymore, and it can be assigned the default value.

I also displayed scenarios in the tests where it's not set and when it's set to a non-default value.

Checklist

• [x] I have reviewed the contributing guidelines.
• [x] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [x] I have updated the documentation.
• [x] I have updated the changelog.