Gateway groups: explain the address requirement of used gateways

[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.

When "Dynamic gateway policy" is used on the interface, no remote address is required when using single gateways. For groups however we can't easily support this as all interfaces inside the group should guaranteed have an address configured to avoid "all pool addresses must be in the same address family" errors.

Describe the solution you like

A clear description of the constraint in the docs (https://docs.opnsense.org/manual/multiwan.html).

Describe alternatives you considered

leaving it as is, possibly fixing the validation when being converted to MVC.

Additional context

• opnsense/core@362b086
• Unable to use a Gateway group (LB/FO) for routing Wireguard VPN traffic. core#6981