Issue Description:

See thread: https://forums.swift.org/t/mixture-of-cantypes-and-siltypes-in-sil/38896 for the original question and answer by John.

Slava's blog posts are related:

1. https://medium.com/@slavapestov/the-secret-life-of-types-in-swift-ff83c3c000a5

2. https://medium.com/@slavapestov/how-to-talk-to-your-kids-about-sil-type-use-6b45f7595f43

which we already mention in ExternalResources.md. Maybe we should mention these resources at specific points in the code or add a brief doc comment explaining the difference and defer to the blog posts for more details?