Interfaces: Virtual IPs: Settings - add unicast carp

[Ad Schellevis]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• 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.

Document new implemented option (opnsense/core#7486)