Document test harness generation
Document test harness generation

Also ensure that we're consistently using the def-site span when
appropriate.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust