Document how to make a project that can use both ENet and WebSockets for high-level multiplayer

**Your Godot version:** 3.2.3

**Issue description:**

We should have a paragraph on making a project that can use both ENet and WebSockets for high-level multiplayer. WebSockets should be used automatically on HTML5 clients since ENet can't be used there. This is because browsers can't send UDP packets for security reasons, and ENet is UDP-only.

The higher latency caveats of WebSockets should also be documented, since WebSockets is TCP-only and therefore forces all RPCs to be reliable. This makes it not well-suited to fast-paced games, but the UDP-friendly alternative (WebRTC) is more complex to set up.

See https://www.reddit.com/r/godot/comments/bux2hs/how_to_use_godots_high_level_multiplayer_api_with/.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/tutorials/networking/high_level_multiplayer.html