Manual: Inform users that `--float` requires `--proto udp`

[TinCanTech]

It is not obvious that Openvpn --float is not possible for TCP protocol.

IE: If the TCP three-way handshake takes place to initiate a new connection, that connection could still --float, if all authentication succeeds. This is how it looks when reading the documentation.

--float is an Openvpn feature, while protocols are an Openvpn dependency.