Document the new NetworkInterface amule.conf setting (bind-to-interface)

[got3nks]

amule-org/amule#281 (closes #173) added a new amule.conf setting that isn't yet on the config reference page:

📄 https://amule-org.github.io/docs/manual/configuration/config-files/amule-conf

Please add an entry for it, alongside the existing Address ("Bind local address to") setting:

Setting	Section	Type	Default	Description
NetworkInterface	[eMule]	string	(empty)	Bind all of aMule's traffic to a specific network interface. Accepts a POSIX interface name (eth0, tun0, en0), a Windows adapter friendly name (Ethernet, Wi-Fi), or a bare interface index; empty means the system default (no binding).

A bit more detail worth capturing in the description:

• Unlike Address (which only sets the source IP), this pins the actual egress interface, so traffic can't leak out via the default route — useful with a VPN.
• It needs no elevated privileges, except on Linux kernels where SO_BINDTODEVICE requires CAP_NET_RAW (aMule logs a warning if the bind can't be applied).
• Set in the GUI via Preferences → Connection → "Bind to network interface". It is a daemon-side setting (not exposed in the remote GUI), and changing it requires a restart.
• Peer-to-peer (ed2k/Kad) traffic is bound on all platforms; HTTP updates (version check, IP2Country, server.met) are bound on Linux/macOS but not on Windows, where they use the system default.