WebSocket Throttling Design discusses how we enforce WebSocket connection throttling. Also contains detailed discussion of how WebSockets integrate with the socket pools. Dates from 2014, but still mostly relevant.
WebSockets over HTTP/2. Current as of 2019 description of WebSocket over H/2 implementation.
WebSocket HTTP Auth Design. This document is very low on detail, but can serve as an overview of how auth works for WebSockets.
Per-renderer WebSocket throttling. While the algorithm described in this document is still used, the code has moved around significantly due to network servicification.