We should better document a few concepts:

1. Filter chains. There is some doc here: https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/listeners/listeners, but it probably needs to be beefed up a bit to talk about independent draining, etc. Please review the docs in this section and see if anything needs updating.
2. LDS filter chain only updates. Probably here: https://www.envoyproxy.io/docs/envoy/latest/configuration/listeners/lds. It would be good to also reference this concept in the arch overview.