Clarify use of flatline query_key values in alerts

[jertel]

Description

Minor doc update for Flatline alerts.

Checklist

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

Questions or Comments

[nsano-rururu]

My understanding is that the following behavior has been added to the documentation. Thank you for your response.
fix a bug where realert doesnt respect query_key when using flatline
Yelp/elastalert#1179

[jertel]

My understanding is that the following behavior has been added to the documentation. Thank you for your response. fix a bug where realert doesnt respect query_key when using flatline Yelp/elastalert#1179

Yes, that's correct. Thanks for reviewing/merging.