man/systemd.netdev: clarify the wireguard AllowedIPs= setting

`AllowedIPs=` only affects "routing inside the network interface
itself", as in, which wireguard peer packets with a specific destination
address are sent to, and what source addresses are accepted from which
peer.

To cause packets to be sent via wireguard in first place, a route via
that interface needs to be added - either in the `[Routes]` section on
the `.network` matching the wireguard interface, or outside of networkd.

This is a common cause of misunderstanding, because tools like wg-quick
also add routes to the interface. However, those tools are meant as a
"extremely simple script for easily bringing up a WireGuard interface,
suitable for a few common use cases (from their manpage).

Networkd also should support other usecases - like setting AllowedIPs to
0.0.0.0/0 and ::/0 and having a dynamic routing protocol setting more
specific routes (or the user manually setting them).

Reported-In: https://github.com/systemd/systemd/issues/14176

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