Adjust firewall rules page to explain multi select rules feature

[Monviech]

Important notices

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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.

https://docs.opnsense.org/manual/how-tos/security-zones.html

Now that this feature is in community and business edition it might be a good choice to update other parts of the documentation like the firewall rules page explanation.

Additional context

https://forum.opnsense.org/index.php?topic=46862.0