stale wireguard-only doc comments in rustdoc

[cachebag]

part of #288. separate from #302 which tracks mdbook guide pages — this is about inline rustdoc comments that still say "wireguard only" or have wireguard-only examples even though openvpn is now supported.

locations

• core/vpn.rs module header (line 8): "Currently supports: WireGuard connections"
• core/vpn.rs various function doc comments still say "WireGuard connection" instead of "VPN connection"
• network_manager.rs connect_vpn (line 273): "Currently supports WireGuard VPN connections" + wireguard-only example
• network_manager.rs list_vpn_connections (line 343): "recognized types (currently WireGuard)"
• api/builders/vpn.rs module doc (line 6): "Currently supports: WireGuard"
• lib.rs quick start (line 35): only shows wireguard example, no openvpn
• lib.rs models doc (line 91): "VpnType - Supported VPN types (WireGuard, etc.)" — should mention openvpn
• core/vpn.rs stale variable name active_wg_map (line 319) — should be active_vpn_map or similar

changes

update all these doc comments to mention openvpn support alongside wireguard. add an openvpn example to connect_vpn docs. rename active_wg_map to active_vpn_map.