Update docs for mix test --slowest and --slowest-modules

[TylerPachal]

I was hunting down a slow test and wasn't sure if --slowest and --slowest-modules considered time spent in setup and setup_all.

After some testing, I found that they both count time spent in setup and neither counts the time spent in setup_all.

Maybe this is obvious to other people, but it wasn't to me, and might be worth adding to the documentation.

---

Here are the updated docs:

[josevalim]

Merged this as it is correct as of now but we should probably include the time of setup_all in --slowest-modules.