Address review comments
Address review comments

- Use prettier syntax for codegen-units defaults
- Remove comment about parallelism that only made sense for specific
values of codegen-units
- Be more specific about what `verbose-tests` does

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