building a mdBook does not print its path unless --open is specified

[lolbinarycat]

when rustdoc finishes building a documentation bundle, it prints its path, which is often nested deep in the build directory.

however, rustbook does not do the same, which is confusing, since they both use the ./x doc command as a frontend.

reproduction: ./x doc src/doc/rustdoc/

it's also worth noting that this information does get printed when you do use --open, so it is easily accessible.