Add more documentation to `bbi_init`: make `.nonmem_dir` argument more clear

[barrettk]

• It became apparent that we needed more clear documentation surrounding the NONMEM installation directory (.nonmem_dir argument), as users were incorrectly passing the directory containing the executable file, rather than the parent directory.

• Added more context to the parameter itself, along with some additional details and an example.

closes https://github.com/metrumresearchgroup/bbr/issues/607

[barrettk]

@kyleam @seth127 I imagine we'll have a little back and forth regarding the documentation updates. Requesting you both to review in the event you both want to give your two cents for added clarity regarding this issue. In all honesty the confusion seems well founded, and I would probably have assumed the same based on the documentation if I hadnt worked with bbr as much. Point being I want to make sure we are as clear as possible, as I would intuitively thing to pass the directory containing the executable, rather than the parent directory. I think the example and details help, but let me know if it's too wordy or not concise enough.

[seth127]

Thanks for all the work on this, both of you. I think this is much better at this point. I especially think showing the potential directory layout will be very helpful. It would be for me, at least.