document "reverse session kill"

[AdSchellevis]

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• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

Our new feature might need some additional documentation (opnsense/core#8548), while there, the kill state feature might need some text as well.
milar