Add inline documentation for lib exported functions

[spalladino]

We've heard from some users that it isn't known that zOS can be used directly from zos-lib. This is especially useful for large projects who already have their build process set up, and they don't want to switch to a different tool, but rather use zOS from their scripts.

Document this usage based on the new zOS lib model from 2.0 once merged.

[eternauta1337]

@spalladino isn't this guide already doing this? https://docs.zeppelinos.org/docs/zos_lib.html

[spalladino]

Updated the title to better reflect what we need to do. We have a guide, but we have no reference for zos-lib. Anyway, we can defer this for later.

[spalladino]

On hold until #1308