Clarify that client's `max` flag doesn't actually do much

The current flag description says:

"capacity for number of multiplexed WebRTC peers"

However, the multiplexing MR (!11 (closed)) has not been merged. The extra Snowflake proxies just time out:

WebRTC: No messages received for 20s -- closing stale connection.
WebRTC: closing DataChannel
WebRTC: closing PeerConnection
WebRTC: DataChannel.OnClose
WebRTC: Closing

I discovered this while writing this post.

Places to update:

• CLI flag description.
• Config struct field.
• Maybe there is another underlying struct that has documentation

Does it have any benefit at the current state?